Semiconductor device including nitride insulating layer and method for manufacturing the same

ABSTRACT

In a transistor including an oxide semiconductor film, movement of hydrogen and nitrogen to the oxide semiconductor film is suppressed. Further, in a semiconductor device using a transistor including an oxide semiconductor film, a change in electrical characteristics is suppressed and reliability is improved. A transistor including an oxide semiconductor film and a nitride insulating film provided over the transistor are included, and an amount of hydrogen molecules released from the nitride insulating film by thermal desorption spectroscopy is less than 5×10 21  molecules/cm 3 , preferably less than or equal to 3×10 21  molecules/cm 3 , more preferably less than or equal to 1×10 21  molecules/cm 3 , and an amount of ammonia molecules released from the nitride insulating film by thermal desorption spectroscopy is less than 1×10 22  molecules/cm 3 , preferably less than or equal to 5×10 21  molecules/cm 3 , more preferably less than or equal to 1×10 21  molecules/cm 3 .

TECHNICAL FIELD

The present invention relates to a semiconductor device including afield-effect transistor and a method for manufacturing the semiconductordevice.

BACKGROUND ART

Transistors used for most flat panel displays typified by a liquidcrystal display device and a light-emitting display device are formedusing silicon semiconductors such as amorphous silicon, single crystalsilicon, and polycrystalline silicon provided over glass substrates.Further, transistors formed using the silicon semiconductors are used inintegrated circuits (ICs) and the like.

In recent years, a technique in which, instead of a siliconsemiconductor, a metal oxide exhibiting semiconductor characteristics isused for transistors has attracted attention. Note that in thisspecification, a metal oxide exhibiting semiconductor characteristics isreferred to as an oxide semiconductor.

For example, a technique is disclosed in which a transistor ismanufactured using zinc oxide or an In—Ga—Zn-based oxide as an oxidesemiconductor and the transistor is used as a switching element or thelike of a pixel of a display device (see Patent Documents 1 and 2).

Meanwhile, it has been pointed out that hydrogen is a source forsupplying carriers particularly in an oxide semiconductor. Therefore,some measures need to be taken to prevent hydrogen from entering theoxide semiconductor at the time of forming the oxide semiconductor.Further, variation in a threshold voltage is suppressed by reducing theamount of hydrogen contained in the oxide semiconductor film or a gateinsulating film in contact with the oxide semiconductor (see PatentDocument 3).

Further, water is a source for supplying hydrogen. Therefore, byproviding a silicon nitride film having a property of blocking waterover a transistor including an oxide semiconductor film, entry of waterfrom the outside to the oxide semiconductor film can be prevented.

REFERENCE Patent Document

[Patent Document 1] Japanese Published Patent Application No.2007-123861

[Patent Document 2] Japanese Published Patent Application No.2007-096055

[Patent Document 3] Japanese Published Patent Application No.2009-224479

DISCLOSURE OF INVENTION

However, nitrogen becomes a source for supplying carriers in a similarmanner to hydrogen. Thus, when nitrogen contained in a silicon nitridefilm enters an oxide semiconductor film, a change in electricalcharacteristics in a transistor including an oxide semiconductor film,typically, a shift of the threshold voltage in the negative directionoccurs. Further, there is a problem in that electrical characteristicsvary among transistors.

Thus, an object of one embodiment of the present invention is tosuppress movement of hydrogen and nitrogen to an oxide semiconductorfilm in a transistor including an oxide semiconductor film. It isanother object of one embodiment of the present invention to suppress achange in electrical characteristics and to improve reliability in asemiconductor device using a transistor including an oxidesemiconductor.

One embodiment of the present invention includes a transistor includingan oxide semiconductor film and a nitride insulating film provided overthe transistor, and an amount of hydrogen molecules released from thenitride insulating film by heating is smaller than 5×10²¹ molecules/cm³,preferably smaller than or equal to 3×10²¹ molecules/cm³, morepreferably smaller than or equal to 1×10²¹ molecules/cm³, and an amountof ammonia molecules released from the nitride insulating film byheating is smaller than 1×10²² molecules/cm³, preferably smaller than orequal to 5×10²¹ molecules/cm³, more preferably smaller than or equal to1×10²¹ molecules/cm³.

Another embodiment of the present invention includes a gate electrode,an oxide semiconductor film overlapping with a part of the gateelectrode with a gate insulating film provided therebetween, a pair ofelectrodes in contact with the oxide semiconductor film, and a nitrideinsulating film provided over the oxide semiconductor film. An amount ofhydrogen molecules released from the nitride insulating film by heatingis smaller than 5×10²¹ molecules/cm³, preferably smaller than or equalto 3×10²¹ molecules/cm³, more preferably smaller than or equal to 1×10²¹molecules/cm³, and an amount of ammonia molecules released from thenitride insulating film by heating is smaller than 1×10²² molecules/cm³,preferably smaller than or equal to 5×10²¹ molecules/cm³, morepreferably smaller than or equal to 1×10²¹ molecules/cm³.

Another embodiment of the present invention includes a nitrideinsulating film as a gate insulating film in a transistor including anoxide semiconductor film. An amount of hydrogen molecules released fromthe nitride insulating film by heating is smaller than 5×10²¹molecules/cm³, preferably smaller than or equal to 3×10²¹ molecules/cm³,more preferably smaller than or equal to 1×10²¹ molecules/cm³, and anamount of ammonia molecules released from the nitride insulating film byheating is smaller than 1×10²² molecules/cm³, preferably smaller than orequal to 5×10²¹ molecules/cm³, more preferably smaller than or equal to1×10²¹ molecules/cm³.

Another embodiment of the present invention includes an oxidesemiconductor film, a pair of electrodes in contact with the oxidesemiconductor film, a gate insulating film provided over at least theoxide semiconductor film, and a gate electrode overlapping with a partof the oxide semiconductor film with the gate insulating film providedtherebetween. The gate insulating film includes a nitride insulatingfilm. An amount of hydrogen molecules released from the nitrideinsulating film by heating is smaller than 5×10²¹ molecules/cm³,preferably smaller than or equal to 3×10²¹ molecules/cm³, morepreferably smaller than or equal to 1×10²¹ molecules/cm³, and an amountof ammonia molecules released from the nitride insulating film byheating is smaller than 1×10²² molecules/cm³, preferably smaller than orequal to 5×10²¹ molecules/cm³, more preferably smaller than or equal to1×10²¹ molecules/cm³.

A nitride insulating film which releases hydrogen molecules less than5×10²¹ molecules/cm³, preferably less than or equal to 3×10²¹molecules/cm³, more preferably less than or equal to 1×10²¹molecules/cm³, and ammonia molecules less than 1×10²² molecules/cm³,preferably less than or equal to 5×10²¹ molecules/cm³, more preferablyless than or equal to 1×10²¹ molecules/cm³ by heating is provided over atransistor including an oxide semiconductor film, whereby the amount ofhydrogen and nitrogen which are moved from the nitride insulating filmto the oxide semiconductor film can be reduced. Further, entry ofhydrogen contained in water to the oxide semiconductor film from theoutside can be suppressed.

Further, as a gate insulating film of a transistor including an oxidesemiconductor film, a nitride insulating film which releases hydrogenmolecules less than 5×10²¹ molecules/cm³, preferably less than or equalto 3×10²¹ molecules/cm³, more preferably less than or equal to 1×10²¹molecules/cm³, and ammonia molecules less than 1×10²² molecules/cm³,preferably less than or equal to 5×10²¹ molecules/cm³, more preferablyless than or equal to 1×10²¹ molecules/cm³ by heating is provided,whereby the amount of hydrogen and nitrogen which are moved from thenitride insulating film to the oxide semiconductor film can be reduced.Further, entry of hydrogen contained in water to the oxide semiconductorfilm from the outside can be suppressed.

With one embodiment of the present invention, a change in electricalcharacteristics of a transistor including an oxide semiconductor film issuppressed and reliability can be improved.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are diagrams illustrating one embodiment of atransistor;

FIGS. 2A to 2D are cross-sectional views illustrating one embodiment ofa method for manufacturing a transistor;

FIGS. 3A and 3B are diagrams illustrating one embodiment of atransistor;

FIGS. 4A to 4D are cross-sectional views illustrating one embodiment ofa method for manufacturing a transistor;

FIG. 5 is a cross-sectional view illustrating one embodiment of atransistor;

FIGS. 6A and 6B are diagrams illustrating one embodiment of atransistor;

FIGS. 7A and 7B are diagrams illustrating one embodiment of atransistor;

FIGS. 8A to 8C are top views illustrating one embodiment of a displaydevice;

FIGS. 9A and 9B are cross-sectional views illustrating one embodiment ofa display device;

FIG. 10 is a cross-sectional view illustrating one embodiment of adisplay device;

FIGS. 11A to 11C are cross-sectional views and a top view illustratingone embodiment of a display device;

FIGS. 12A and 12B illustrate one embodiment of a semiconductor device;

FIGS. 13A to 13C each illustrate an electronic device;

FIGS. 14A to 14C illustrate an electronic device;

FIGS. 15A and 15B each illustrate a structure of a sample;

FIGS. 16A to 16C show results of TDS analysis;

FIGS. 17A and 17B show results of TDS analysis;

FIGS. 18A and 18B show results of TDS analysis;

FIGS. 19A and 19B show results of TDS analysis;

FIGS. 20A to 20C each show Vg-Id characteristics of a transistor;

FIGS. 21A to 21C each show Vg-Id characteristics of a transistor;

FIGS. 22A to 22C each show Vg-Id characteristics of a transistor; and

FIG. 23 shows an amount of released hydrogen molecules and an amount ofreleased ammonia molecules from a silicon nitride film, and Vg-Idcharacteristics of a transistor.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. However, the presentinvention is not limited to the following description and it is easilyunderstood by those skilled in the art that the mode and details can bevariously changed without departing from the scope and spirit of thepresent invention. Therefore, the present invention should not beconstrued as being limited to the description in the followingembodiments and examples. In addition, in the following embodiments andexamples, the same portions or portions having similar functions aredenoted by the same reference numerals or the same hatching patterns indifferent drawings, and description thereof will not be repeated.

Note that in each drawing described in this specification, the size, thefilm thickness, or the region of each component is exaggerated forclarity in some cases. Therefore, embodiments of the present inventionare not limited to such scales.

Note that terms such as “first”, “second”, and “third” in thisspecification are used in order to avoid confusion among components, andthe terms do not limit the components numerically. Therefore, forexample, the term “first” can be replaced with the term “second”,“third”, or the like as appropriate.

Functions of a “source” and a “drain” are sometimes replaced with eachother when the direction of current flow is changed in circuitoperation, for example. Therefore, the terms “source” and “drain” can beused to denote the drain and the source, respectively, in thisspecification.

Note that a voltage refers to a difference between potentials of twopoints, and a potential refers to electrostatic energy (electricpotential energy) of a unit charge at a given point in an electrostaticfield. Note that in general, a difference between potential of one pointand reference potential (e.g., ground potential) is merely calledpotential or voltage, and potential and voltage are used as synonymouswords in many cases. Thus, in this specification, a potential may berephrased as a voltage and a voltage may be rephrased as a potentialunless otherwise specified.

Note that a transistor including an oxide semiconductor film is ann-channel transistor; therefore, in this specification, a transistorwhich can be regarded as having no drain current flowing therein when agate voltage is 0 V is defined as a transistor having normally-offcharacteristics. In contrast, a transistor which can be regarded ashaving a drain current flowing therein when a gate voltage is 0 V isdefined as a transistor having normally-on characteristics.

(Embodiment 1)

In this embodiment, a semiconductor device of one embodiment of thepresent invention and a method for manufacturing the semiconductordevice is described with reference to drawings.

FIGS. 1A and 1B are a top view and a cross-sectional view of atransistor 1 included in a semiconductor device. FIG. 1A is a top viewof the transistor 1 and FIG. 1B is a cross-sectional view taken alongdashed-dotted line A-B in FIG. 1A. Note that in FIG. 1A, a substrate 11,some components of the transistor 1 (e.g., a gate insulating film 18),an insulating film 23, a nitride insulating film 25, and the like areomitted for simplicity.

The transistor 1 illustrated in FIGS. 1A and 1B includes a gateelectrode 15 provided over the substrate 11, the gate insulating film 18formed over the substrate 11 and the gate electrode 15, an oxidesemiconductor film 19 overlapping with the gate electrode 15 with thegate insulating film 18 provided therebetween, and a pair of electrodes21 in contact with the oxide semiconductor film 19. A protective film 26including the insulating film 23 and the nitride insulating film 25 isformed over the gate insulating film 18, the oxide semiconductor film19, and the pair of electrodes 21.

The nitride insulating film 25 provided over the transistor 1 in thisembodiment releases hydrogen molecules less than 5×10²¹ molecules/cm³,preferably less than or equal to 3×10²¹ molecules/cm³, more preferablyless than or equal to 1×10²¹ molecules/cm³, and ammonia molecules lessthan 1×10²² molecules/cm³, preferably less than or equal to 5×10²¹molecules/cm³, more preferably less than or equal to 1×10²¹molecules/cm³ by thermal desorption spectroscopy (TDS). The number ofhydrogen molecules and the number of ammonia molecules which are sourcesfor supplying nitrogen, which are released from the nitride insulatingfilm 25, are small; thus, the amount of hydrogen and nitrogen which aremoved to the oxide semiconductor film 19 in the transistor 1 is small.

Hydrogen contained in the oxide semiconductor film 19 reacts with oxygenbonded to a metal atom to produce water, and a defect is formed in alattice from which oxygen is released (or a portion from which oxygen isreleased). In addition, part of hydrogen reacts with oxygen, whichcauses generation of electrons serving as carriers. Further, nitrogencontained in the oxide semiconductor film 19 reacts with a metal elementor oxygen, which causes generation of electrons serving as carriers. Asa result, a transistor including the oxide semiconductor film 19 tendsto be normally on. Therefore, hydrogen and nitrogen in the oxidesemiconductor film 19 are reduced as much as possible, whereby a shiftof the threshold voltage in the negative direction can be suppressed andvariation in electrical characteristics can be reduced. Further, leakagecurrent between a source and a drain of the transistor, typicallyoff-state current, can be reduced.

A nitride insulating film which releases hydrogen molecules less than5×10²¹ molecules/cm³, preferably less than or equal to 3×10²¹molecules/cm³, more preferably less than or equal to 1×10²¹molecules/cm³, and ammonia molecules less than 1×10²² molecules/cm³,preferably less than or equal to 5×10²¹ molecules/cm³, more preferablyless than or equal to 1×10²¹ molecules/cm³ by thermal desorptionspectroscopy is provided over the transistor 1, whereby the amount ofhydrogen and ammonia which are moved from the nitride insulating film tothe oxide semiconductor film 19 can be small and concentration ofhydrogen and nitrogen in the oxide semiconductor film 19 can be reduced.Further, the nitride insulating film 25 is provided over the transistor1; therefore, entry of water from the outside to the oxide semiconductorfilm 19 can be suppressed. In other words, entry of hydrogen containedin water to the oxide semiconductor film 19 can be suppressed. As aresult, a shift of the threshold voltage in the negative direction canbe suppressed and variation in electrical characteristics can bereduced. Further, leakage current between a source and a drain of thetransistor, typically off-state current, can be reduced.

As the nitride insulating film 25, silicon nitride, silicon nitrideoxide, aluminum nitride, aluminum nitride oxide, or the like with athickness greater than or equal to 50 nm and less than or equal to 200nm can be used. Note that in this specification, a “silicon oxynitridefilm” refers to a film that contains more oxygen than nitrogen, and a“silicon nitride oxide film” refers to a film that contains morenitrogen than oxygen. Further, an “aluminum oxynitride film” refers to afilm that contains more oxygen than nitrogen, and an “aluminum nitrideoxide film” refers to a film that contains more nitrogen than oxygen.

Here, a method to measure the number of released hydrogen molecules andreleased ammonia molecules using thermal desorption spectroscopy(hereinafter, referred to as TDS analysis) is described below.

The amount of released gas in the TDS analysis is proportional to anintegral value of spectrum. Therefore, the amount of released gas can becalculated from the ratio between the integral value of a spectrum of aninsulating film and the reference value of a standard sample. Thereference value of a standard sample refers to the ratio of the densityof a predetermined atom contained in a sample to the integral value of aspectrum.

For example, the number of the released hydrogen molecules (N_(H2)) froman insulating film can be calculated according to Formula 1 using theTDS analysis results of a silicon wafer containing hydrogen at apredetermined density, which is the standard sample, and the TDSanalysis results of the insulating film. Here, all spectra having a massnumber of 2 which are obtained by the TDS analysis are assumed tooriginate from an hydrogen molecule. Further, an isotope of a hydrogenatom having a mass number other than 1 is not taken into considerationbecause the proportion of such a molecule in the natural world isminimal

$\begin{matrix}\left\lbrack {{FORMULA}\mspace{14mu} 1} \right\rbrack & \; \\{\mspace{281mu}{N_{H\; 2} = {\frac{N_{H\; 2{(S)}}}{S_{H\; 2{(S)}}} \times S_{H\; 2} \times \alpha}}} & (1)\end{matrix}$

N_(H2) is the number of the released hydrogen molecules. N_(H2(S)) isthe value obtained by conversion of the number of hydrogen moleculesdesorbed from the standard sample into densities. S_(H2(S)) is theintegral value of a spectrum when the standard sample is subjected toTDS analysis. Here, the reference value of the standard sample is set toN_(H2(S))/S_(H2(S)). S_(H2) is the integral value of a spectrum when theinsulating film is subjected to TDS analysis. α is a coefficientaffecting the intensity of the spectrum in the TDS analysis. Refer toJapanese Published Patent Application No. H6-275697 for details ofFormula 1. Note that the number of released hydrogen molecules from theabove insulating film is measured with a thermal desorption spectroscopyapparatus produced by ESCO Ltd., EMD-WA1000S/W using a silicon wafercontaining hydrogen atoms at 1×10¹⁶ atoms/cm³ as the standard sample.

Further, in Formula 1, an integral value of spectrum when the number ofreleased ammonia molecules from an insulating film is measured by theTDS analysis is substituted into S_(H2), whereby the number of releasedammonia molecules can be obtained.

Other details of the transistor 1 are described below.

There is no particular limitation on the property of a material and thelike of the substrate 11 as long as the material has heat resistanceenough to withstand at least later heat treatment. For example, a glasssubstrate, a ceramic substrate, a quartz substrate, a sapphiresubstrate, or the like may be used as the substrate 11. Alternatively, asingle crystal semiconductor substrate or a polycrystallinesemiconductor substrate made of silicon, silicon carbide, or the like, acompound semiconductor substrate made of silicon germanium or the like,an SOI (silicon on insulator) substrate, or the like may be used as thesubstrate 11. Furthermore, any of these substrates further provided witha semiconductor element may be used as the substrate 11.

Still alternatively, a flexible substrate may be used as the substrate11, and the transistor 1 may be provided directly on the flexiblesubstrate. Alternatively, a separation layer may be provided between thesubstrate 11 and the transistor 1. The separation layer can be used whenpart or the whole of a semiconductor device formed over the separationlayer is separated from the substrate 11 and transferred onto anothersubstrate. In such a case, the transistor 1 can move to a substratehaving low heat resistance or a flexible substrate as well.

The base insulating film may be provided between the substrate 11 andthe gate electrode 15. As the base insulating film, a silicon oxidefilm, a silicon oxynitride film, a silicon nitride film, a siliconnitride oxide film, a gallium oxide film, a hafnium oxide film, anyttrium oxide film, an aluminum oxide film, an aluminum oxynitride film,and the like can be given as examples. Note that when a silicon nitridefilm, a gallium oxide film, a hafnium oxide film, an yttrium oxide film,an aluminum oxide film, or the like is used as the base insulating film,it is possible to suppress diffusion of impurities, typically, an alkalimetal, water, hydrogen, and the like, into the oxide semiconductor film19 from the substrate 11.

The gate electrode 15 can be formed using a metal element selected fromaluminum, chromium, copper, tantalum, titanium, molybdenum, andtungsten; an alloy containing any of these metal elements as acomponent; an alloy containing these metal elements in combination; orthe like. Further, one or more metal elements selected from manganeseand zirconium may be used. Further, the gate electrode 15 may have asingle-layer structure or a stacked-layer structure of two or morelayers. A single-layer structure of an aluminum film containing silicon;a two-layer structure in which a titanium film is stacked over analuminum film; a two-layer structure in which a titanium film is stackedover a titanium nitride film; a two-layer structure in which a tungstenfilm is stacked over a titanium nitride film; a two-layer structure inwhich a tungsten film is stacked over a tantalum nitride film or atungsten nitride film; and a three-layer structure in which a titaniumfilm, an aluminum film, and a titanium film are stacked in this ordercan be given as examples. Alternatively, a film, an alloy film, or anitride film which contains aluminum and one or more elements selectedfrom titanium, tantalum, tungsten, molybdenum, chromium, neodymium, andscandium may be used.

The gate electrode 15 can also be formed using a light-transmittingconductive material such as indium tin oxide, indium oxide containingtungsten oxide, indium zinc oxide containing tungsten oxide, indiumoxide containing titanium oxide, indium tin oxide containing titaniumoxide, indium zinc oxide, or indium tin oxide including silicon oxide.It is also possible to have a stacked-layer structure formed using theabove light-transmitting conductive material and the above metalelement.

Further, an In—Ga—Zn-based oxynitride semiconductor film, an In—Sn-basedoxynitride semiconductor film, an In—Ga-based oxynitride semiconductorfilm, an In—Zn-based oxynitride semiconductor film, a Sn-basedoxynitride semiconductor film, an In-based oxynitride semiconductorfilm, a metal nitride film (such as an InN film or a ZnN film), or thelike may be provided between the gate electrode 15 and the gateinsulating film 18. These films each have a work function higher than orequal to 5 eV, preferably higher than or equal to 5.5 eV, which ishigher than the electron affinity of an oxide semiconductor; thus, thethreshold voltage of a transistor including the oxide semiconductor canbe shifted in the positive direction. Accordingly, a switching elementhaving what is called normally-off characteristics can be obtained. Forexample, in the case of using an In—Ga—Zn-based oxynitride semiconductorfilm, an In—Ga—Zn-based oxynitride semiconductor film having a highernitrogen concentration than at least the oxide semiconductor film 19,specifically, an In—Ga—Zn-based oxynitride semiconductor film having anitrogen concentration higher than or equal to 7 at. %, is used.

The gate insulating film 18 can be formed to have a single-layerstructure or a stacked-layer structure using, for example, one or moreof a silicon oxide film, a silicon oxynitride film, a silicon nitrideoxide film, a silicon nitride film, an aluminum oxide film, a hafniumoxide film, a gallium oxide film, and a Ga—Zn-based metal oxide film.Note that in order to improve the characteristics of the interfacebetween the gate insulating film 18 and the oxide semiconductor film 19,a region in the gate insulating film 18 which is in contact with atleast the oxide semiconductor film 19 is preferably formed using anoxide insulating film.

It is possible to prevent outward diffusion of oxygen from the oxidesemiconductor film 19 and entry of hydrogen, water, or the like into theoxide semiconductor film 19 from the outside by providing an insulatingfilm having a blocking effect against oxygen, hydrogen, water, and thelike for the gate insulating film 18. As for the insulating film havinga blocking effect against oxygen, hydrogen, water, and the like, analuminum oxide film, an aluminum oxynitride film, a gallium oxide film,a gallium oxynitride film, an yttrium oxide film, an yttrium oxynitridefilm, a hafnium oxide film, and a hafnium oxynitride film can be givenas examples.

Further, the gate insulating film 18 has a stacked structure in which afirst silicon nitride film is formed using a silicon nitride film havingfewer defects, a second silicon nitride film using a silicon nitridefilm which releases the small number of hydrogen molecules and ammoniamolecules as the nitride insulating film 25 is formed over the firstsilicon nitride film, and an oxide insulating film is formed over thesecond silicon nitride film, whereby the gate insulating film 18 can beformed using a gate insulating film which has fewer defects and releasesthe small number of hydrogen molecules and ammonia molecules. As aresult, movement of hydrogen and nitrogen contained in the gateinsulating film 18 to the oxide semiconductor film 19 can be suppressed.

By using a silicon nitride film as the gate insulating film 18, thefollowing effect can be obtained. The silicon nitride film has a higherrelative permittivity than a silicon oxide film and needs a largerthickness for an equivalent capacitance. Thus, the physical thickness ofthe gate insulating film can be increased. This makes it possible tosuppress a decrease in withstand voltage of the transistor 1 andfurthermore improve the withstand voltage, thereby suppressingelectrostatic discharge damage to a semiconductor device.

Further, in the case where copper is used for the gate electrode 15 anda silicon nitride film is used as the gate insulating film 18 in contactwith the gate electrode 15, as the gate insulating film 18, a siliconnitride film releasing ammonia molecules by heating, which are reducedas much as possible, is preferably used. Thus, as the silicon nitridefilm, a silicon nitride film which can be used as the nitride insulatingfilm 25 can be used. As a result, reaction between copper and ammoniamolecules can be suppressed.

In the case where the trap level (also referred to as interface level)is present at the interface between an oxide semiconductor film and agate insulating film or in the gate insulating film in a transistorusing an oxide semiconductor, a shift of the threshold voltage of thetransistor, typically, a shift of the threshold voltage in the negativedirection, and an increase in the subthreshold swing (S value) showing agate voltage needed for changing the drain current by one digit when thetransistor is turned on are caused. As a result, there is a problem inthat electrical characteristics vary among transistors. Therefore, byusing a silicon nitride film having fewer defects is used as a gateinsulating film, a shift of the threshold voltage in the negativedirection and variation in electrical characteristics of the transistorcan be reduced.

The gate insulating film 18 may be formed using a high-k material suchas hafnium silicate (HfSiO_(x)), hafnium silicate to which nitrogen isadded (HfSi_(x)O_(y)N_(z)), hafnium aluminate to which nitrogen is added(HfAl_(x)O_(y)N_(z)), hafnium oxide, or yttrium oxide, so that gateleakage current of the transistor can be reduced.

The thickness of the gate insulating film 18 is preferably greater thanor equal to 5 nm and less than or equal to 400 nm, more preferablygreater than or equal to 10 nm and less than or equal to 300 nm, stillmore preferably greater than or equal to 50 nm and less than or equal to250 nm.

The oxide semiconductor film 19 preferably contains at least indium (In)or zinc (Zn). Alternatively, the oxide semiconductor film 19 preferablycontains both In and Zn. In order to reduce variation in electricalcharacteristics of the transistors including the oxide semiconductorfilm 19, the oxide semiconductor film 19 preferably contains one or moreof stabilizers in addition to In or Zn.

As for stabilizers, gallium (Ga), tin (Sn), hafnium (Hf), aluminum (Al),zirconium (Zr), and the like can be given. As another stabilizer,lanthanoids such as lanthanum (La), cerium (Ce), praseodymium (Pr),neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium(Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm),ytterbium (Yb), lutetium (Lu), and the like can be given.

As the oxide semiconductor, for example, the following can be used:indium oxide, tin oxide; zinc oxide; a two-component metal oxide such asan In—Zn-based metal oxide, a Sn—Zn-based metal oxide, an Al—Zn-basedmetal oxide, a Zn—Mg-based metal oxide, a Sn—Mg-based metal oxide, anIn—Mg-based metal oxide, an In—Ga-based metal oxide, or an In—W-basedmetal oxide; a three-component metal oxide such as an In—Ga—Zn-basedmetal oxide (also referred to as an IGZO), an In—Al—Zn-based metaloxide, an In—Sn—Zn-based metal oxide, a Sn—Ga—Zn-based metal oxide, anAl—Ga—Zn-based metal oxide, a Sn—Al—Zn-based metal oxide, anIn—Hf—Zn-based metal oxide, an In—La—Zn-based metal oxide, anIn—Ce—Zn-based metal oxide, an In—Pr—Zn-based metal oxide, anIn—Nd—Zn-based metal oxide, an In—Sm—Zn-based metal oxide, anIn—Eu—Zn-based metal oxide, an In—Gd—Zn-based metal oxide, anIn—Tb—Zn-based metal oxide, an In—Dy—Zn-based metal oxide, anIn—Ho—Zn-based metal oxide, an In—Er—Zn-based metal oxide, anIn—Tm—Zn-based metal oxide, an In—Yb—Zn-based metal oxide, or anIn—Lu—Zn-based metal oxide; or a four-component metal oxide such as anIn—Sn—Ga—Zn-based metal oxide, an In—Hf—Ga—Zn-based metal oxide, anIn—Al—Ga—Zn-based metal oxide, an In—Sn—Al—Zn-based metal oxide, anIn—Sn—Hf—Zn-based metal oxide, or an In—Hf—Al—Zn-based metal oxide.

Note that, for example, an In—Ga—Zn-based metal oxide means an oxidecontaining In, Ga, and Zn as its main components and there is noparticular limitation on the ratio of In, Ga, and Zn. The In—Ga—Zn-basedmetal oxide may contain a metal element other than In, Ga, and Zn.

Alternatively, a material represented by InMO₃(ZnO)_(m) (m>0 issatisfied, and m is not an integer) may be used as the oxidesemiconductor. Note that M represents one or more metal elementsselected from Ga, Fe, Mn, and Co. Alternatively, as the oxidesemiconductor, a material represented by In₂SnO₅(ZnO)_(n) (n>0 issatisfied, n is an integer) may be used.

For example, it is possible to use an In—Ga—Zn-based metal oxidecontaining In, Ga, and Zn at an atomic ratio of 1:1:1 (=⅓:⅓:⅓), 2:2:1(=⅖:⅖:⅕), or 3:1:2 (=½:⅙:⅓), or any of oxides whose composition is inthe neighborhood of the above compositions. Alternatively, anIn—Sn—Zn-based metal oxide containing In, Sn, and Zn at an atomic ratioof 1:1:1 (=⅓:⅓:⅓), 2:1:3 (=⅓:⅙:½), or 2:1:5 (=¼:⅛:⅝) may be used. Notethat a proportion of each atom in the atomic ratio of the metal oxidevaries within a range of ±20% as an error.

However, the composition is not limited to those described above, and amaterial having the appropriate composition may be used depending onneeded semiconductor characteristics and electrical characteristics(e.g., field-effect mobility, threshold voltage, and variation). Inorder to obtain needed semiconductor characteristics, it is preferablethat the carrier density, the impurity concentration, the defectdensity, the atomic ratio of a metal element and oxygen, the interatomicdistance, the density, and the like be set to be appropriate.

For example, a high mobility can be obtained relatively easily in thecase where the In—Sn—Zn-based metal oxide is used. However, the mobilitycan be increased by reducing the defect density in the bulk also in thecase where the In—Ga—Zn-based metal oxide is used.

Note that the energy gap of a metal oxide that can form the oxidesemiconductor film 19 is 2 eV or more, preferably 2.5 eV or more,further preferably 3 eV or more. In this manner, the off-state currentof a transistor can be reduced by using an oxide semiconductor having awide energy gap.

Note that the oxide semiconductor film 19 may have an amorphousstructure, a single crystal structure, or a polycrystalline structure.

As the oxide semiconductor film 19, a c-axis aligned crystalline oxidesemiconductor film (also referred to as a CAAC-OS film) having crystalparts may be used.

The CAAC-OS film is one of oxide semiconductor films including aplurality of crystal parts, and most of each crystal part fits inside acube whose one side is less than 100 nm. Thus, there is a case where acrystal part included in the CAAC-OS film fits a cube whose one side isless than 10 nm, less than 5 nm, or less than 3 nm. The density ofdefect states of the CAAC-OS film is lower than that of themicrocrystalline oxide semiconductor film. The CAAC-OS film is describedin detail below.

In a transmission electron microscope (TEM) image of the CAAC-OS film, aboundary between crystal parts, that is, a grain boundary is not clearlyobserved. Thus, in the CAAC-OS film, a reduction in electron mobilitydue to the grain boundary is less likely to occur.

According to the TEM image of the CAAC-OS film observed in a directionsubstantially parallel to a sample surface (cross-sectional TEM image),metal atoms are arranged in a layered manner in the crystal parts. Eachmetal atom layer has a morphology reflected by a surface over which theCAAC-OS film is formed (hereinafter, a surface over which the CAAC-OSfilm is formed is referred to as a formation surface) or a top surfaceof the CAAC-OS film, and is arranged in parallel to the formationsurface or the top surface of the CAAC-OS film.

On the other hand, according to the TEM image of the CAAC-OS filmobserved in a direction substantially perpendicular to the samplesurface (plan TEM image), metal atoms are arranged in a triangular orhexagonal configuration in the crystal parts. However, there is noregularity of arrangement of metal atoms between different crystalparts.

From the results of the cross-sectional TEM image and the plan TEMimage, alignment is found in the crystal parts in the CAAC-OS film.

A CAAC-OS film is subjected to structural analysis with an X-raydiffraction (XRD) apparatus. For example, when the CAAC-OS filmincluding an InGaZnO₄ crystal is analyzed by an out-of-plane method, apeak appears frequently when the diffraction angle (2θ) is around 31°.This peak is derived from the (009) plane of the InGaZnO₄ crystal, whichindicates that crystals in the CAAC-OS film have c-axis alignment, andthat the c-axes are aligned in a direction substantially perpendicularto the formation surface or the top surface of the CAAC-OS film.

On the other hand, when the CAAC-OS film is analyzed by an in-planemethod in which an X-ray enters a sample in a direction perpendicular tothe c-axis, a peak appears frequently when 2θ is around 56°. This peakis derived from the (110) plane of the InGaZnO₄ crystal. Here, analysis(ϕ scan) is performed under conditions where the sample is rotatedaround a normal vector of a sample surface as an axis (ϕ axis) with 2θfixed at around 56°. In the case where the sample is a single-crystaloxide semiconductor film of InGaZnO₄, six peaks appear. The six peaksare derived from crystal planes equivalent to the (110) plane. On theother hand, in the case of a CAAC-OS film, a peak is not clearlyobserved even when ϕ scan is performed with 2θ fixed at around 56°.

According to the above results, in the CAAC-OS film having c-axisalignment, while the directions of a-axes and b-axes are differentbetween crystal parts, the c-axes are aligned in a direction parallel toa normal vector of a formation surface or a normal vector of a topsurface. Thus, each metal atom layer arranged in a layered mannerobserved in the cross-sectional TEM image corresponds to a planeparallel to the a-b plane of the crystal.

Note that the crystal part is formed concurrently with deposition of theCAAC-OS film or is formed through crystallization treatment such as heattreatment. As described above, the c-axis of the crystal is aligned witha direction parallel to a normal vector of a formation surface or anormal vector of a top surface of the CAAC-OS film. Thus, for example,in the case where a shape of the CAAC-OS film is changed by etching orthe like, the c-axis might not be necessarily parallel to the normalvector of the formation surface or the normal vector of the top surfaceof the CAAC-OS film.

Further, the degree of crystallinity in the CAAC-OS film is notnecessarily uniform. For example, in the case where crystal growthleading to the CAAC-OS film occurs from the vicinity of the top surfaceof the film, the degree of the crystallinity in the vicinity of the topsurface is higher than that in the vicinity of the formation surface insome cases. Further, when an impurity is added to the CAAC-OS film, thecrystallinity in a region to which the impurity is added is changed, andthe degree of crystallinity in the CAAC-OS film varies depending onregions.

Note that when the CAAC-OS film with an InGaZnO₄ crystal is analyzed byan out-of-plane method, a peak of 2θ may also be observed at around 36°,in addition to the peak of 2θ at around 31°. The peak of 2θ at around36° indicates that a crystal having no c-axis alignment is included inpart of the CAAC-OS film. It is preferable that in the CAAC-OS film, apeak of 2θ appear at around 31° and a peak of 2θ do not appear at around36°.

In a transistor using the CAAC-OS film, a change in electricalcharacteristics due to irradiation with visible light or ultravioletlight is small. Thus, the transistor has high reliability.

Alternatively, the oxide semiconductor film 19 may have a stacked-layerstructure of a plurality of oxide semiconductor films. For example, theoxide semiconductor film 19 may have a stacked-layer structure of afirst oxide semiconductor film and a second oxide semiconductor filmwhich are formed using metal oxides with different compositions.Alternatively, for example, the first oxide semiconductor film may beformed using any of a two-component metal oxide, a three-component metaloxide, and a four-component metal oxide, and the second oxidesemiconductor film may be formed using any of these which is differentfrom the oxide for the first oxide semiconductor film.

Further, the constituent elements of the first oxide semiconductor filmand the second oxide semiconductor film may be made the same and thecomposition of the constituent elements of the first oxide semiconductorfilm and the second oxide semiconductor film may be made different. Forexample, the first oxide semiconductor film may contain In, Ga, and Znat an atomic ratio of 3:1:2, and the second oxide semiconductor film maycontain In, Ga, and Zn at an atomic ratio of 1:1:1. Alternatively, thefirst oxide semiconductor film may contain In, Ga, and Zn at an atomicratio of 2:1:3, and the second oxide semiconductor film may contain In,Ga, and Zn at an atomic ratio of 1:3:2. Note that a proportion of eachatom in the atomic ratio of the oxide semiconductor varies within arange of ±20% as an error.

At this time, one of the first oxide semiconductor film and the secondoxide semiconductor film, which is closer to the gate electrode (on thechannel side), preferably contains In and Ga at a proportion of In>Ga.The other oxide semiconductor film, which is farther from the gateelectrode (on the back channel side) preferably contains In and Ga at aproportion of In≤Ga.

Further, the oxide semiconductor film 19 may have a three-layerstructure of a first oxide semiconductor film, a second oxidesemiconductor film, and a third oxide semiconductor film, in which theconstituent elements thereof is made the same and the composition of theconstituent elements of the first oxide semiconductor film, the secondoxide semiconductor film, and the third oxide semiconductor film is madedifferent. For example, the first oxide semiconductor film may containIn, Ga, and Zn at an atomic ratio of 1:3:2, the second oxidesemiconductor film may contain In, Ga, and Zn at an atomic ratio of3:1:2, and the third oxide semiconductor film may contain In, Ga, and Znat an atomic ratio of 1:1:1.

As compared to an oxide semiconductor film containing more In than Gaand Zn at an atomic ratio, typically, the second oxide semiconductorfilm, and an oxide semiconductor film containing Ga, Zn, and In at thesame atomic ratio, typically, the third oxide semiconductor film, anoxide semiconductor film which contains less In than Ga and Zn at anatomic ratio, typically, the first oxide semiconductor film containingIn, Ga, and Zn at an atomic ratio of 1:3:2, has few oxygen vacancies,and thus can suppress an increase in carrier density. Further, when thefirst oxide semiconductor film containing In, Ga, and Zn at an atomicratio of 1:3:2 has an amorphous structure, the second oxidesemiconductor film is likely to be a CAAC-OS film.

Since the constituent elements of the first oxide semiconductor film,the second oxide semiconductor film, and the third oxide semiconductorfilm are the same, the first oxide semiconductor film has fewer traplevels at the interface with the second oxide semiconductor film.Therefore, when the oxide semiconductor film 19 has the above structure,the amount of change in threshold voltage of the transistor due to achange over time or a BT photostress test can be reduced.

In an oxide semiconductor, the s orbital of heavy metal mainlycontributes to carrier transfer, and when the In content in the oxidesemiconductor is increased, overlap of the s orbitals is likely to beincreased. Therefore, an oxide containing In and Ga at a proportion ofIn>Ga has higher carrier mobility than an oxide containing In and Ga ata proportion of In≤Ga. Further, in Ga, the formation energy of an oxygenvacancy is larger and thus an oxygen vacancy is less likely to occur,than in In; therefore, the oxide containing In and Ga at a proportion ofIn≤Ga has more stable characteristics than the oxide containing In andGa at a proportion of In>Ga.

An oxide semiconductor containing In and Ga at a proportion of In>Ga isused on the channel side, and an oxide semiconductor containing In andGa at a proportion of In≤Ga is used on the back channel side, so thatthe field-effect mobility and the reliability of the transistor can befurther improved.

Further, the first oxide semiconductor film, the second oxidesemiconductor film, and the third oxide semiconductor film may be formedusing oxide semiconductors having different crystallinity. That is,these oxide semiconductor films may be formed using any of a singlecrystal oxide semiconductor, a polycrystalline oxide semiconductor, anamorphous oxide semiconductor, and a CAAC-OS, as appropriate. When anamorphous oxide semiconductor is used for either the first oxidesemiconductor film or the second oxide semiconductor film, internalstress or external stress of the oxide semiconductor film 19 isrelieved, variation in characteristics of the transistor is reduced, andthe reliability of the transistor can be further improved.

The thickness of the oxide semiconductor film 19 is preferably greaterthan or equal to 1 nm and less than or equal to 100 nm, more preferablygreater than or equal to 1 nm and less than or equal to 30 nm, stillmore preferably greater than or equal to 1 nm and less than or equal to50 nm, further preferably greater than or equal to 3 nm and less than orequal to 20 nm.

The concentration of alkali metals or alkaline earth metals in the oxidesemiconductor film 19, which is obtained by secondary ion massspectrometry (SIMS), is preferably lower than or equal to 1×10¹⁸atoms/cm³, more preferably lower than or equal to 2×10¹⁶ atoms/cm³. Thisis because, when alkali metals or alkaline earth metals are bonded to anoxide semiconductor, some of the alkali metals or the alkaline earthmetals generate carriers and cause an increase in the off-state currentof the transistor.

In the oxide semiconductor film 19, the hydrogen concentration obtainedby secondary ion mass spectrometry is preferably smaller than 5×10¹⁸atoms/cm³, further preferably smaller than or equal to 1×10¹⁸ atoms/cm³,still further preferably smaller than or equal to 5×10¹⁷ atoms/cm³, yetstill further preferably smaller than or equal to 1×10¹⁶ atoms/cm³.

Hydrogen contained in the oxide semiconductor film 19 reacts with oxygenbonded to a metal atom to produce water, and a defect is formed in alattice from which oxygen is released (or a portion from which oxygen isremoved). In addition, a bond of part of hydrogen and oxygen causesgeneration of electrons serving as a carrier. Thus, the impuritiescontaining hydrogen are reduced as much as possible in the step offorming the oxide semiconductor film, whereby the hydrogen concentrationin the oxide semiconductor film can be reduced. Therefore, an oxidesemiconductor film in which hydrogen is removed as much as possible isused in a channel formation region, whereby a shift of the thresholdvoltage in the negative direction can be suppressed and variation inelectrical characteristics can be reduced. Further, leakage currentbetween a source and a drain of the transistor, typically off-statecurrent, can be reduced.

In addition, the nitrogen concentration in the oxide semiconductor film19 is set to be lower than or equal to 5×10¹⁸ atoms/cm³, whereby a shiftof the threshold voltage in the negative direction can be suppressed andvariation in electrical characteristics can be reduced.

Various experiments can prove low off-state current of a transistorincluding a highly-purified oxide semiconductor film from which hydrogenis removed as much as possible as a channel formation region. Forexample, even when an element has a channel width of 1×10⁶ μm and achannel length of 10 μm, off-state current can be less than or equal tothe measurement limit of a semiconductor parameter analyzer, i.e., lessthan or equal to 1×10⁻¹³ A, at voltage (drain voltage) between thesource electrode and the drain electrode of from 1 V to 10 V. In thiscase, it can be seen that the off-state current is 100 zA/μm or lower.Further, the off-state current was measured with the use of a circuit inwhich a capacitor and a transistor are connected to each other andcharge that flows in or out from the capacitor is controlled by thetransistor. In the measurement, a purified oxide semiconductor film hasbeen used for a channel formation region of the transistor, and theoff-state current of the transistor has been measured from a change inthe amount of charge of the capacitor per unit time. As a result, it isfound that in the case where the voltage between the source electrodeand the drain electrode of the transistor is 3 V, lower off-statecurrent of several tens of yoctoamperes per micrometer (yA/μm) can beobtained. Consequently, the transistor including the highly purifiedoxide semiconductor film as the channel formation region has extremelysmall off-state current.

The pair of electrodes 21 is formed to have a single-layer structure ora stacked-layer structure including, as a conductive material, any ofmetals such as aluminum, titanium, chromium, nickel, copper, yttrium,zirconium, molybdenum, silver, tantalum, and tungsten or an alloycontaining any of these metals as its main component. A single-layerstructure of an aluminum film containing silicon; a two-layer structurein which a titanium film is stacked over an aluminum film; a two-layerstructure in which a titanium film is stacked over a tungsten film; atwo-layer structure in which a copper film is formed over acopper-magnesium-aluminum alloy film; a three-layer structure in which atitanium film or a titanium nitride film, an aluminum film or a copperfilm, and a titanium film or a titanium nitride film are stacked in thisorder; and a three-layer structure in which a molybdenum film or amolybdenum nitride film, an aluminum film or a copper film, and amolybdenum film or a molybdenum nitride film are stacked in this ordercan be given as examples. Note that a transparent conductive materialcontaining indium oxide, tin oxide, or zinc oxide may be used.

Although the pair of electrodes 21 is provided between the oxidesemiconductor film 19 and the insulating film 23 in this embodiment, thepair of electrodes 21 may be provided between the gate insulating film18 and the oxide semiconductor film 19.

In order to improve the characteristics of the interface between theinsulating film 23 and the oxide semiconductor film 19, an oxideinsulating film is preferably used as the insulating film 23. As theinsulating film 23, silicon oxide, silicon oxynitride, aluminum oxide,hafnium oxide, gallium oxide, Ga—Zn-based metal oxide, or the like witha thickness more than or equal to 150 nm and less than or equal to 400nm can be used.

Next, a method for manufacturing the transistor 1 illustrated in FIGS.1A and 1B is described with reference to FIGS. 2A to 2D.

As illustrated in FIG. 2A, the gate electrode 15 is formed over thesubstrate 11, and the gate insulating film 18 is formed over the gateelectrode 15.

A formation method of the gate electrode 15 is described below. First, aconductive film is formed by a sputtering method, a CVD method, anevaporation method, or the like and then a mask is formed over theconductive film by a photolithography process. Then, part of theconductive film is etched using the mask to form the gate electrode 15.After that, the mask is removed.

Note that instead of the above formation method, the gate electrode 15may be formed by an electrolytic plating method, a printing method, anink-jet method, or the like.

Here, a 100-nm-thick tungsten film is formed by a sputtering method.Then, a mask is formed by a photolithography process and the tungstenfilm is dry-etched using the mask to form the gate electrode 15.

The gate insulating film 18 is formed by a sputtering method, a CVDmethod, an evaporation method, or the like.

In the case where the gate insulating film 18 is formed using a siliconoxide film or a silicon oxynitride film, a deposition gas containingsilicon and an oxidizing gas are preferably used as a source gas. Astypical examples of the deposition gas containing silicon, silane,disilane, trisilane, and silane fluoride can be given. As the oxidizinggas, oxygen, ozone, dinitrogen monoxide, nitrogen dioxide, and the likecan be given as examples.

Further, in the case where a stacked structure of a silicon nitride filmand an oxide insulating film is formed as the gate insulating film 18,the silicon nitride film is preferably stacked by a two-step formationmethod. First, a first silicon nitride film with few defects is formedby a plasma CVD method in which a mixed gas of silane, nitrogen, andammonia is used as a source gas. Then, by using a source gas at a flowrate ratio which is similar to that of a source gas used for the nitrideinsulating film 25 described later, a silicon nitride film whichreleases the small number of hydrogen molecules and ammonia moleculescan be formed as the second silicon nitride film. With such a formationmethod, a silicon nitride film which has few defects and releases thesmall number of hydrogen molecules and ammonia molecules can be formedas the gate insulating film 18.

Moreover, in the case where a gallium oxide film is formed as the gateinsulating film 18, a metal organic chemical vapor deposition (MOCVD)method can be used.

Here, by a plasma CVD method, the gate insulating film 18 in which thefirst silicon nitride film with a thickness of 300 nm, the secondsilicon nitride film with a thickness of 50 nm, and the siliconoxynitride film with a thickness of 50 nm are stacked is formed.

Next, as illustrated in FIG. 2B, an oxide semiconductor film 19 isformed over the gate insulating film 18.

A formation method of the oxide semiconductor film 19 is describedbelow. An oxide semiconductor film is formed over the gate insulatingfilm 18 by a sputtering method, a coating method, a pulsed laserdeposition method, a laser ablation method, or the like. Then, after amask is formed over the oxide semiconductor film by a photolithographyprocess, the oxide semiconductor film is partly etched using the mask.Accordingly, the oxide semiconductor film 19 which is over the gateinsulating film 18 and subjected to element isolation so as to partlyoverlap with the gate electrode 15 is formed as illustrated in FIG. 2B.After that, the mask is removed.

Alternatively, by using a printing method for forming the oxidesemiconductor film 19, the oxide semiconductor film 19 subjected toelement isolation can be formed directly.

In the case where the oxide semiconductor film is formed by a sputteringmethod, a power supply device for generating plasma can be an RF powersupply device, an AC power supply device, a DC power supply device, orthe like as appropriate.

As a sputtering gas, a rare gas (typically argon), an oxygen gas, or amixed gas of a rare gas and oxygen is used as appropriate. In the caseof using the mixed gas of a rare gas and oxygen, the proportion ofoxygen is preferably higher than that of a rare gas.

Further, a target may be appropriately selected in accordance with thecomposition of the oxide semiconductor film to be formed.

For example, in the case where the oxide semiconductor film is formed bya sputtering method at a substrate temperature higher than or equal to150° C. and lower than or equal to 750° C., preferably higher than orequal to 150° C. and lower than or equal to 450° C., more preferablyhigher than or equal to 200° C. and lower than or equal to 350° C., theoxide semiconductor film can be a CAAC-OS film.

A CAAC-OS film is formed by, for example, a sputtering method using apolycrystalline oxide semiconductor sputtering target. When ions collidewith the sputtering target, a crystal region included in the sputteringtarget might be separated from the target along an a-b plane; in otherwords, a sputtered particle having a plane parallel to an a-b plane(flat-plate-like sputtered particle or pellet-like sputtered particle)might be separated from the sputtering target. In that case, theflat-plate-like sputtered particle reaches a substrate while maintainingtheir crystal state, whereby the CAAC-OS film can be deposited.

For the deposition of the CAAC-OS film, the following conditions arepreferably used.

By suppressing the number of impurities entering the CAAC-OS film duringthe deposition, the crystal state can be prevented from being broken bythe impurities. For example, reducing the concentration of impurities(e.g., hydrogen, water, carbon dioxide, and nitrogen) which exist in thedeposition chamber is favorable. Furthermore, the concentration ofimpurities in a deposition gas can be reduced. Specifically, adeposition gas whose dew point is lower than or equal to −80° C.,preferably lower than or equal to −100° C., can be used.

By increasing the substrate heating temperature during the deposition,migration of a sputtered particle is likely to occur after the sputteredparticle reaches a substrate surface. Specifically, the substrateheating temperature during the deposition is higher than or equal to100° C. and lower than the strain point of the substrate, preferablyhigher than or equal to 200° C. and lower than or equal to 500° C. Byincreasing the substrate heating temperature during the deposition, whenthe flat-plate-like sputtered particle reaches the substrate, migrationoccurs on the substrate surface, so that a flat plane of theflat-plate-like sputtered particle is attached to the substrate.

Furthermore, it is preferable that the proportion of oxygen in thedeposition gas be increased and the power be optimized in order toreduce plasma damage at the deposition. The proportion of oxygen in thedeposition gas is higher than or equal to 30 vol %, preferably 100 vol%.

As an example of the sputtering target, an In—Ga—Zn-based metal oxidetarget is described below.

The In—Ga—Zn-based metal oxide target, which is polycrystalline, is madeby mixing InO_(X) powder, GaO_(Y) powder, and ZnO_(Z) powder in apredetermined molar ratio, applying pressure, and performing heattreatment at a temperature higher than or equal to 1000° C. and lowerthan or equal to 1500° C. Note that X, Y, and Z are each a givenpositive number. Here, the predetermined molar ratio of InO_(X) powderto GaO_(Y) powder and ZnO_(Z) powder is, for example, 2:2:1, 8:4:3,3:1:1, 1:1:1, 4:2:3, or 3:1:2. The kinds of powder and the molar ratiofor mixing powder may be determined as appropriate depending on thedesired sputtering target.

Further, after the oxide semiconductor film is formed, heat treatmentmay be performed so that the oxide semiconductor film is subjected todehydrogenation or dehydration. The heating temperature is typicallyhigher than or equal to 150° C. and lower than the strain point of thesubstrate, preferably higher than or equal to 250° C. and lower than orequal to 450° C., more preferably higher than or equal to 300° C. andlower than or equal to 450° C.

The heat treatment is performed under an inert gas atmosphere containingnitrogen or a rare gas such as helium, neon, argon, xenon, or krypton.Alternatively, the heat treatment may be performed under an inert gasatmosphere first, and then under an oxygen atmosphere. It is preferablethat the above inert gas atmosphere and the above oxygen atmosphere donot contain hydrogen, water, and the like. The treatment time is 3minutes to 24 hours.

An electric furnace, an RTA apparatus, or the like can be used for theheat treatment. With the use of an RTA apparatus, the heat treatment canbe performed at a temperature of higher than or equal to the strainpoint of the substrate if the heating time is short. Therefore, the heattreatment time can be shortened.

By performing heat treatment after the oxide semiconductor film isformed, in the oxide semiconductor film, the concentration of hydrogencan be smaller than 5×10¹⁸ atoms/cm³, preferably smaller than or equalto 1×10¹⁸ atoms/cm³, further preferably smaller than or equal to 5×10¹⁷atoms/cm³, still further preferably smaller than or equal to 1×10¹⁶atoms/cm³.

Here, a 35-nm-thick oxide semiconductor film is formed by a sputteringmethod, a mask is formed over the oxide semiconductor film, and thenpart of the oxide semiconductor film is selectively etched. Next, afterthe mask is removed, heat treatment is performed in an atmosphere ofnitrogen and oxygen, so that the oxide semiconductor film 19 is formed.

Next, as illustrated in FIG. 2C, the pair of electrodes 21 is formed.

A formation method of the pair of electrodes 21 is described below.First, a conductive film is formed by a sputtering method, a CVD method,an evaporation method, or the like. Then, a mask is formed over theconductive film by a photolithography process. After that, theconductive film is etched using the mask to form the pair of electrodes21. Then, the mask is removed.

Here, a 50-nm-thick tungsten film, a 400-nm-thick aluminum film, and a100-nm-thick titanium film are sequentially stacked by a sputteringmethod. Then, a mask is formed over the titanium film by aphotolithography process and the tungsten film, the aluminum film, andthe titanium film are dry-etched using the mask to form the pair ofelectrodes 21.

Note that heat treatment may be performed after the pair of electrodes21 is formed. This heat treatment can be performed in a manner similarto that of the heat treatment performed after the oxide semiconductorfilm 19 is formed.

After the pair of electrodes 21 is formed, cleaning treatment ispreferably performed to remove an etching residue. A short circuit ofthe pair of electrodes 21 can be suppressed by this cleaning treatment.The cleaning treatment can be performed using an alkaline solution suchas a tetramethylammonium hydroxide (TMAH) solution; an acidic solutionsuch as a hydrofluoric acid solution or an oxalic acid solution; orwater.

Next, the insulating film 23 is formed over the oxide semiconductor film19 and the pair of electrodes 21. The insulating film 23 can be formedby a sputtering method, a CVD method, an evaporation method, or thelike.

Here, as the insulating film 23, a silicon oxide film or a siliconoxynitride film is formed by a plasma CVD method.

Next, heat treatment may be performed. The temperature of the heattreatment is typically higher than or equal to 150° C. and lower thanthe strain point of the substrate, preferably higher than or equal to200° C. and lower than or equal to 450° C., more preferably higher thanor equal to 300° C. and lower than or equal to 450° C. By the heattreatment, water, hydrogen, and the like contained in the insulatingfilm 23 can be released.

Here, the heat treatment is performed at 350° C. for one hour in anatmosphere of nitrogen and oxygen.

Next, the nitride insulating film 25 is formed over the insulating film23. The nitride insulating film 25 can be formed by a sputtering method,a CVD method, or the like.

In the case where a silicon nitride film is formed by the plasma CVDmethod as the nitride insulating film 25, a deposition gas containingsilicon, nitrogen, and ammonia is used as the source gas. As the sourcegas, a small amount of ammonia compared to the amount of nitrogen isused, whereby ammonia is dissociated in the plasma and activated speciesare generated. The activated species cleave a bond between silicon andhydrogen which are contained in a deposition gas containing silicon anda triple bond between nitrogen molecules. As a result, a dense siliconnitride film having few defects, in which a bond between silicon andnitrogen is promoted and a bond between silicon and hydrogen is few canbe formed. On the other hand, when the amount of ammonia with respect tonitrogen is large in a source gas, cleavage of a deposition gascontaining silicon and cleavage of nitrogen are not promoted, so that asparse silicon nitride film in which a bond between silicon and hydrogenremains and defects are increased is formed. Therefore, in a source gas,a flow ratio of the nitrogen to the ammonia is set to be greater than orequal to 5 and less than or equal to 50, preferably greater than orequal to 10 and less than or equal to 50.

Here, in a reaction chamber of a plasma CVD apparatus, the 50-nm-thicksilicon nitride film is formed by a plasma CVD method under thefollowing conditions: silane with a flow rate of 50 sccm, nitrogen witha flow rate of 5000 sccm, and ammonia with a flow rate of 100 sccm areused as a source gas, the pressure in the treatment chamber is 200 Pa,the substrate temperature is 220° C., and high-frequency power of 1000 Wis supplied to parallel-plate electrodes with a high-frequency powersupply of 27.12 MHz. Note that the plasma CVD apparatus is aparallel-plate plasma CVD apparatus in which the electrode area is 6000cm², and the power per unit area (power density) into which the suppliedpower is converted is 1.7×10⁻¹ W/cm².

Through the above steps, a protective film 26 including the insulatingfilm 23 and the nitride insulating film 25 which releases the smallnumber of hydrogen molecules and ammonia molecules can be formed.

Next, heat treatment may be performed. The temperature of the heattreatment is typically higher than or equal to 150° C. and lower thanthe strain point of the substrate, preferably higher than or equal to200° C. and lower than or equal to 450° C., more preferably higher thanor equal to 300° C. and lower than or equal to 450° C.

Through the above steps, a nitride insulating film which releases thesmall amount of hydrogen molecules and ammonia molecules can be formedover a transistor including an oxide semiconductor film. Further, atransistor with the improved reliability, in which a change inelectrical characteristics is suppressed, can be manufactured.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments andexamples.

(Embodiment 2)

In this embodiment, a transistor having a structure different from thatof Embodiment 1 will be described with reference to FIGS. 3A and 3B. Atransistor 3 shown in this embodiment is a top-gate transistor, which isdifferent from the transistors in Embodiment 1.

FIGS. 3A and 3B are a top view and a cross-sectional view of thetransistor 3. FIG. 3A is a top view of the transistor 3, and FIG. 3B isa cross-sectional view taken along dashed-dotted line A-B in FIG. 3A.Note that in FIG. 3A, a substrate 31, a base insulating film 33, somecomponents of the transistor 3 (e.g., an insulating film 37 and anitride insulating film 39), and the like are omitted for simplicity.

The transistor 3 illustrated in FIGS. 3A and 3B includes an oxidesemiconductor film 34 over the base insulating film 33, a pair ofelectrodes 35 in contact with the oxide semiconductor film 34, a gateinsulating film 40 in contact with the base insulating film 33, theoxide semiconductor film 34, and the pair of electrodes 35, and a gateelectrode 41 overlapping with the oxide semiconductor film 34 with thegate insulating film 40 provided therebetween.

The gate insulating film 40 provided in the transistor 3 in thisembodiment includes an insulating film 37 and a nitride insulating film39. As the insulating film 37, the oxide insulating film used as thegate insulating film 18 in Embodiment 1 is used as appropriate, wherebythe interface state between the oxide semiconductor film 34 and the gateinsulating film 40 can be reduced. As the nitride insulating film 39, anitride insulating film like the nitride insulating film 25 inEmbodiment 1 can be used, which releases hydrogen molecules less than5×10²¹ molecules/cm³, preferably less than or equal to 3×10²¹molecules/cm³, more preferably less than or equal to 1×10²¹molecules/cm³, and ammonia molecules less than 1×10²² molecules/cm³,preferably less than or equal to 5×10²¹ molecules/cm³, more preferablyless than or equal to 1×10²¹ molecules/cm³ by thermal desorptionspectroscopy. The number of hydrogen molecules and the number of ammoniamolecules which are released from the nitride insulating film 39 aresmall; thus, the amount of hydrogen and nitrogen which are moved to theoxide semiconductor film 34 in the transistor 3 is small.

As a result, in the transistor 3, the amount of hydrogen and nitrogenwhich are moved from the gate insulating film 40 to the oxidesemiconductor film 34 is small and the concentration of hydrogen andnitrogen in the oxide semiconductor film 34 can be reduced. Further, thenitride insulating film 39 is included in the gate insulating film inthe transistor 3, whereby entry of water from the outside to the oxidesemiconductor film 34 can be suppressed. In other words, entry ofhydrogen contained in water to the oxide semiconductor film 34 can besuppressed. As a result, a shift of the threshold voltage in thenegative direction can be suppressed and variation in electricalcharacteristics can be reduced. Further, leakage current between asource and a drain of the transistor, typically off-state current, canbe reduced.

Hereinafter, other details of the transistor 3 will be described.

As the substrate 31, a substrate which is given as an example of thesubstrate 11 in Embodiment 1 can be used as appropriate.

As the base insulating film 33, an oxide insulating film whose oxygencontent is in excess of that in the stoichiometric composition ispreferably used. The oxide insulating film whose oxygen content is inexcess of that in the stoichiometric composition can diffuse oxygen intoan oxide semiconductor film by heat treatment. Typical examples of thebase insulating film 33 are films of silicon oxide, silicon oxynitride,silicon nitride oxide, gallium oxide, hafnium oxide, yttrium oxide,aluminum oxide, aluminum oxynitride, and the like.

The thickness of the base insulating film 33 is greater than or equal to50 nm, preferably greater than or equal to 200 nm and less than or equalto 3000 nm, more preferably greater than or equal to 300 nm and lessthan or equal to 1000 nm. With use of the thick base insulating film 33,the number of oxygen molecules released from the base insulating film 33can be increased, and the interface state density at the interfacebetween the base insulating film 33 and an oxide semiconductor filmformed later can be reduced.

Here, “to release part of oxygen by heating” means that the amount ofreleased oxygen by conversion into oxygen atoms is greater than or equalto 1×10¹⁸ atoms/cm³, preferably greater than or equal to 3×10²⁰atoms/cm³ in TDS analysis.

The oxide semiconductor film 34 can be formed in a manner similar tothat of the oxide semiconductor film 19 in Embodiment 1.

The pair of electrodes 35 can be formed in a manner similar to that ofthe pair of electrodes 21 shown in Embodiment 1. Note that the length ofthe pair of electrodes 35 in the channel width direction is larger thanthat of the oxide semiconductor film 34, and seen in the cross sectionin the channel length direction, the pair of electrodes 35 covers endportions of the oxide semiconductor film 34. With such a structure, thearea of contact between the pair of electrodes 35 and the oxidesemiconductor film 34 is increased. Thus, the contact resistance betweenthe oxide semiconductor film 34 and the pair of electrodes 35 can bereduced, and the on-state current of the transistor can be increased.

In this embodiment, the pair of electrodes 35 is provided between theoxide semiconductor film 34 and the insulating film 37; however, thepair of electrodes 35 may be provided between the base insulating film33 and the oxide semiconductor film 34.

Further, the insulating film 23 and the nitride insulating film 25 areprovided over the gate insulating film 40 and the gate electrode 41 inthe same manner as in Embodiment 1, whereby entry of water from theoutside to the transistor 3 including the oxide semiconductor film canbe further suppressed.

The gate electrode 41 can be formed in a manner similar to that of thegate electrode 15 in Embodiment 1.

Next, a method for manufacturing the transistor illustrated in FIGS. 3Aand 3B will be described with reference to FIGS. 4A to 4D.

As illustrated in FIG. 4A, the base insulating film 33 is formed overthe substrate 31. Next, the oxide semiconductor film 34 is formed overthe base insulating film 33.

The base insulating film 33 is formed by a sputtering method, a CVDmethod or the like.

When the oxide insulating film from which part of oxygen is released byheating is formed by a sputtering method as the base insulating film 33,the amount of oxygen in a deposition gas is preferably large, andoxygen, a mixed gas of oxygen and a rare gas, or the like can be used.Typically, the oxygen concentration of a deposition gas is preferablyfrom 6% to 100%.

In the case where an oxide insulating film is formed by a CVD method asthe base insulating film 33, hydrogen or water derived from a source gasis sometimes mixed in the oxide insulating film. Thus, after the oxideinsulating film is formed by a CVD method, heat treatment is preferablyperformed as dehydrogenation or dehydration.

In the case of adding oxygen to the oxide insulating film formed by aCVD method, the amount of oxygen released by heating can be increased.As the method for adding oxygen to the oxide insulating film, an ionimplantation method, an ion doping method, a plasma immersion ionimplantation method, plasma treatment, or the like is used.

The oxide semiconductor film 34 can be formed as appropriate by aformation method similar to that of the oxide semiconductor film 19described in Embodiment 1.

In order to improve the orientation of the crystal parts in the CAAC-OSfilm, planarity of the surface of the base insulating film 33 serving asa base insulating film of the oxide semiconductor film is preferablyimproved. Typically, the base insulating film 33 can be made to have anaverage surface roughness (Ra) of 1 nm or less, 0.3 nm or less, or 0.1nm or less. Note that Ra is obtained by expanding, into threedimensions, the arithmetic mean surface roughness defined by JIS B 0601so that it can be applied to a curved surface, and Ra can be expressedas an “average value of the absolute values of deviations from areference surface to a specific surface” and is defined by Formula 2.

$\begin{matrix}{\left\lbrack {{FORMULA}\mspace{14mu} 2} \right\rbrack\mspace{574mu}} & \; \\{{{Ra} = {\frac{1}{S_{0}}{\int_{y\; 1}^{y\; 2}{\int_{x\; 1}^{x\; 2}{{{{f\left( {x,y} \right)} - Z_{0}}}{dxdy}}}}}}\ } & (2)\end{matrix}$

Here, the specific surface is a surface which is a target of roughnessmeasurement, and is a quadrilateral region which is specified by fourpoints represented by the coordinates (x₁, y₁, f(x₁, y₁)), (x₁, y₂,f(x₁, y₂)), (x₂, y₁, y₁)), and (x₂, y₂, f(x₂, y₂)). S₀ represents thearea of a rectangle which is obtained by projecting the specific surfaceon the xy plane, and Z₀ represents the height of the reference surface(the average height of the specific surface). Ra can be measured usingan atomic force microscope (AFM).

As planarization treatment for improving planarity of the surface of thebase insulating film 33, one or more can be selected from chemicalmechanical polishing (CMP) treatment, dry etching treatment, plasmatreatment (reverse sputtering), and the like. The plasma treatment isthe one in which minute unevenness of the surface is reduced byintroducing an inert gas such as an argon gas into a vacuum chamber andapplying an electric field so that a surface to be processed serves as acathode.

Next, heat treatment is preferably performed. By this heat treatment,part of oxygen contained in the base insulating film 33 can be diffusedto the vicinity of the interface between the base insulating film 33 andthe oxide semiconductor film 34. As a result, the interface state in thevicinity of the interface between the base insulating film 33 and theoxide semiconductor film 34 can be reduced.

The temperature of the heat treatment is typically higher than or equalto 150° C. and lower than the strain point of the substrate, preferablyhigher than or equal to 250° C. and lower than or equal to 450° C., morepreferably higher than or equal to 300° C. and lower than or equal to450° C.

The heat treatment is performed under an inert gas atmosphere containingnitrogen or a rare gas such as helium, neon, argon, xenon, or krypton.Alternatively, the heat treatment may be performed under an inert gasatmosphere first, and then under an oxygen atmosphere. It is preferablethat the above inert gas atmosphere and the above oxygen atmosphere donot contain hydrogen, water, and the like. The treatment time is 3minutes to 24 hours.

Next, as illustrated in FIG. 4B, the pair of electrodes 35 is formed.The pair of electrodes 35 can be formed as appropriate by a formationmethod similar to that of the pair of electrodes 21 described inEmbodiment 1. Alternatively, the pair of electrodes 35 can be formed bya printing method or an inkjet method.

Next, as illustrated in FIG. 4C, the insulating film 37 and the nitrideinsulating film 39 included in the gate insulating film 40 are formed.

The insulating film 37 is formed by a sputtering method, a CVD method,an evaporation method, or the like.

As the nitride insulating film 39, in a similar manner to the nitrideinsulating film 25 in Embodiment 1, a nitride insulating film whichreleases hydrogen molecules less than 5×10²¹ molecules/cm³, preferablyless than or equal to 3×10²¹ molecules/cm³, more preferably less than orequal to 1×10²¹ molecules/cm³, and ammonia molecules less than 1×10²²molecules/cm³, preferably less than or equal to 5×10²¹ molecules/cm³,more preferably less than or equal to 1×10²¹ molecules/cm³ by thermaldesorption spectroscopy can be formed.

Next, as illustrated in FIG. 4D, the gate electrode 41 is formed overthe gate insulating film 40. The gate electrode 41 can be formed asappropriate by a formation method similar to that of the gate electrode15 in Embodiment 1.

Next, in a manner similar to that in Embodiment 1, heat treatment may beperformed. The temperature of the heat treatment is typically higherthan or equal to 150° C. and lower than the strain point of thesubstrate, preferably higher than or equal to 250° C. and lower than orequal to 450° C., more preferably higher than or equal to 300° C. andlower than or equal to 450° C.

Through the above steps, a transistor with the improved reliability, inwhich a change in electrical characteristics is suppressed, can bemanufactured.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments andexamples.

(Embodiment 3)

In this embodiment, a transistor having a different structure from thetransistors in Embodiment 1 and Embodiment 2 will be described withreference to FIG. 5. A transistor 5 of this embodiment includes aplurality of gate electrodes facing each other with an oxidesemiconductor film provided therebetween.

The transistor 5 illustrated in FIG. 5 includes the gate electrode 15provided over the substrate 11, the gate insulating film 18 formed overthe substrate 11 and the gate electrode 15, the oxide semiconductor film19 overlapping with the gate electrode 15 with the gate insulating film18 provided therebetween, and the pair of electrodes 21 in contact withthe oxide semiconductor film 19. The protective film 26 including theinsulating film 23 and the nitride insulating film 25 is formed over thegate insulating film 18, the oxide semiconductor film 19, and the pairof electrodes 21. Further, a gate electrode 61 overlapping with theoxide semiconductor film 19 with the protective film 26 providedtherebetween is included.

The gate electrode 61 can be formed in a manner similar to that of thegate electrode 15 in Embodiment 1.

The transistor 5 of this embodiment has the gate electrode 15 and thegate electrode 61 facing each other with the oxide semiconductor film 19provided therebetween. By application of different potentials to thegate electrode 15 and the gate electrode 61, the threshold voltage ofthe transistor 5 can be controlled. Alternatively, when the samepotential is applied to the gate electrode 15 and the gate electrode 61,the on-state current of the transistor 5 can be increased. A nitrideinsulating film which releases hydrogen molecules less than 5×10²¹molecules/cm³, preferably less than or equal to 3×10²¹ molecules/cm³,more preferably less than or equal to 1×10²¹ molecules/cm³, and ammoniamolecules less than 1×10²² molecules/cm³, preferably less than or equalto 5×10²¹ molecules/cm³, more preferably less than or equal to 1×10²¹molecules/cm³ by thermal desorption spectroscopy is provided between theoxide semiconductor film 19 and the gate electrode 61, whereby theamount of hydrogen and ammonia which are moved from the nitrideinsulating film to the oxide semiconductor film 19 can be small andconcentration of hydrogen and nitrogen in the oxide semiconductor film19 can be reduced. Further, the nitride insulating film 25 is providedbetween the oxide semiconductor film 19 and the gate electrode 61;therefore, entry of water from the outside to the oxide semiconductorfilm 19 can be suppressed. In other words, entry of hydrogen containedin water to the oxide semiconductor film 19 can be suppressed. As aresult, a shift of the threshold voltage in the negative direction canbe suppressed and variation in electrical characteristics can bereduced.

(Embodiment 4)

In this embodiment, a structure of a transistor and a protective film inwhich movement of hydrogen and nitrogen to an oxide semiconductor filmis suppressed and oxygen vacancies in the oxide semiconductor film canbe reduced will be described with reference to FIGS. 6A and 6B. Notethat the description about the same structures as those in Embodiment 1will be omitted.

In a transistor using an oxide semiconductor, oxygen vacancies in anoxide semiconductor film cause defects of electrical characteristics ofthe transistor. For example, the threshold voltage of a transistor usingan oxide semiconductor film with oxygen vacancies easily shifts in thenegative direction, and such a transistor tends to be normally-on. Thisis because electric charges are generated owing to oxygen vacancies inthe oxide semiconductor, and the resistance is reduced.

Further, when the oxide semiconductor film includes an oxygen vacancy,as a problem, the amount of change in electrical characteristics,typically, the threshold voltage of the transistor is increased due to achange over time or a bias-temperature stress test (hereinafter alsoreferred to as a BT stress test).

In this embodiment, a transistor with excellent electricalcharacteristics in which a shift of the threshold voltage in thenegative direction is suppressed and a manufacturing method thereof aredescribed. In addition, a highly reliable transistor in which variationin electrical characteristics due to a change over time or a BTphotostress test is small and a manufacturing method thereof aredescribed.

FIGS. 6A and 6B are a top view and a cross-sectional view of atransistor 7 included in a semiconductor device. FIG. 6A is a top viewof the transistor 7, and FIG. 6B is a cross-sectional view taken alongdashed line A-B in FIG. 6A. Note that in FIG. 6A, the substrate 11, somecomponents of the transistor 7 (e.g., the gate insulating film 18), aninsulating film 24 a, an insulating film 24 b, the nitride insulatingfilm 25, a planarization film 27, and the like are omitted forsimplicity.

The transistor 7 illustrated in FIGS. 6A and 6B includes the gateelectrode 15 provided over the substrate 11, the gate insulating film 18formed over the substrate 11 and the gate electrode 15, an oxidesemiconductor film 19 overlapping with the gate electrode 15 with thegate insulating film 18 provided therebetween, and a pair of electrodes21 in contact with the oxide semiconductor film 19. A protective film 28including the insulating film 24 a, the insulating film 24 b, and thenitride insulating film 25 is formed over the gate insulating film 18,the oxide semiconductor film 19, and the pair of electrodes 21. Inaddition, the planarization film 27 may be provided over the protectivefilm 28. Moreover, in an opening 30 formed in the protective film 28 andthe planarization film 27, a conductive film 29 connected to one of thepair of electrodes 21 may be provided.

In the transistor 7 shown in this embodiment, the insulating film 24 ais formed in contact with the oxide semiconductor film 19. Theinsulating film 24 a is an oxide insulating film which transmits oxygen.Note that the insulating film 24 a also functions as a film whichrelieves damage to the oxide semiconductor film 19 at the time offorming the insulating film 24 b later.

As the oxide insulating film which transmits oxygen, a silicon oxidefilm, a silicon oxynitride film, or the like having a thickness greaterthan or equal to 5 nm and less than or equal to 150 nm, preferablygreater than or equal to 5 nm and less than or equal to 50 nm, morepreferably greater than or equal to 10 nm and less than or equal to 30nm can be used.

Further, it is preferable that the number of defects be small in theinsulating film 24 a, and typically, the spin density of a signal atg=2.001 which is due to dangling bonds of silicon by ESR measurement belower than or equal to 3×10¹⁷ spins/cm³, further preferably lower thanor equal to 5×10¹⁶ spins/cm³. This is because, when defect density inthe insulating film 24 a is high, oxygen may be bonded to the defect andthe transmittance of oxygen in the insulating film 24 a is decreased.

Further, it is preferable that the number of defects be small at aninterface between the insulating film 24 a and the oxide semiconductorfilm 19, and typically, the spin density of a signal at g=1.93 which isdue to oxygen vacancies in the oxide semiconductor film by ESRmeasurement in which a magnetic field is applied in parallel to thesurface of the film be lower than or equal to 1×10¹⁷ spins/cm³, furtherpreferably lower than or equal to a lower limit of the detection. Thespin density due to oxygen vacancies in the oxide semiconductor film 19is lower than or equal to the above-described spin density, whereby inVg-Id characteristics in the transistor including the oxidesemiconductor film, variation in the gate voltage at which thetransistor is turned on in the case where there is different drainvoltages can be reduced.

Note that all oxygen atoms entering the insulating film 24 a from theoutside are not moved to the outside of the insulating film 24 a andsome oxygen remains in the insulating film 24 a in some cases. Further,oxygen enters the insulating film 24 a and oxygen contained in theinsulating film 24 a is moved to the outside of the insulating film 24a, whereby movement of oxygen in the insulating film 24 a occurs in somecases.

By forming the oxide insulating film which transmits oxygen as theinsulating film 24 a, oxygen released from the oxide insulating filmprovided over the insulating film 24 a, whose oxygen content is inexcess of that in the stoichiometric composition, can be moved to theoxide semiconductor film 19 through the insulating film 24 a.

The insulating film 24 b is formed to be in contact with the insulatingfilm 24 a. The insulating film 24 b is formed using an oxide insulatingfilm whose oxygen content is in excess of that in the stoichiometriccomposition. Such an oxide insulating film whose oxygen content is inexcess of that in the stoichiometric composition is an oxide insulatingfilm from which part of oxygen is released by heating. The oxideinsulating film whose oxygen content is in excess of that in thestoichiometric composition is, similarly to the base insulating film 33in Embodiment 2, an oxide insulating film in which the amount ofreleased oxygen by conversion into oxygen atoms is greater than or equalto 1×10¹⁸ atoms/cm³, preferably greater than or equal to 3×10²⁰atoms/cm³ in TDS analysis.

As the insulating film 24 a, a silicon oxide film or a siliconoxynitride film can be formed under the following conditions: thesubstrate placed in a treatment chamber of the plasma CVD apparatus,which is vacuum-evacuated, is held at a temperature higher than or equalto 180° C. and lower than or equal to 400° C., preferably higher than orequal to 200° C. and lower than or equal to 370° C., the pressure in thetreatment chamber is greater than or equal to 30 Pa and less than orequal to 250 Pa, preferably greater than or equal to 40 Pa and less thanor equal to 200 Pa with introduction of a source gas into the treatmentchamber, and high-frequency power is supplied to an electrode providedin the treatment chamber.

Note that when the ratio of the amount of the oxidizing gas to theamount of the deposition gas containing silicon is 100 or higher, thehydrogen content in the insulating film 24 a can be reduced.Consequently, the amount of hydrogen entering the insulating film 24 acan be reduced; thus, the shift of the threshold voltage of thetransistor in the negative direction can be suppressed.

As the insulating film 24 b, a silicon oxide film, a silicon oxynitridefilm, or the like having a thickness greater than or equal to 30 nm andless than or equal to 500 nm, preferably greater than or equal to 50 nmand less than or equal to 400 nm can be used.

Further, it is preferable that the insulating film 24 b have a fewdefects, and typically, the spin density of a signal at g=2.001 which isdue to dangling bonds of silicon by ESR measurement be lower than orequal to 1×10¹⁸ spins/cm³. Note that the insulating film 24 b isprovided more apart from the oxide semiconductor film 19 than theinsulating film 24 a is; thus, the insulating film 24 b may have moredefect density than the insulating film 24 a.

As the insulating film 24 b, a silicon oxide film or a siliconoxynitride film is formed under the following conditions: the substrateplaced in a treatment chamber of the plasma CVD apparatus, which isvacuum-evacuated, is held at a temperature higher than or equal to 180°C. and lower than or equal to 260° C., preferably higher than or equalto 180° C. and lower than or equal to 240° C., the pressure is greaterthan or equal to 100 Pa and less than or equal to 250 Pa, preferablygreater than or equal to 100 Pa and less than or equal to 200 Pa withintroduction of a source gas into the treatment chamber, andhigh-frequency power higher than or equal to 0.17 W/cm² and lower thanor equal to 0.5 W/cm², preferably higher than or equal to 0.25 W/cm² andlower than or equal to 0.35 W/cm² is supplied to an electrode providedin the treatment chamber.

As the film formation conditions of the insulating film 24 b, thehigh-frequency power having the above power density is supplied to thetreatment chamber having the above pressure, whereby the degradationefficiency of the source gas in plasma is increased, oxygen radicals areincreased, and oxidation of the source gas is promoted; therefore, theoxygen content of the insulating film 24 b becomes higher than that inthe stoichiometric composition. On the other hand, in the film formed ata substrate temperature within the above temperature range, the bondingstrength between silicon and oxygen is weak, and accordingly, part ofoxygen in the film is released by heating in the later step. Thus, it ispossible to form an oxide insulating film whose oxygen content is inexcess of that in the stoichiometric composition and from which part ofoxygen is released by heating. Moreover, the insulating film 24 a isprovided over the oxide semiconductor film 19. In the formation step ofthe insulating film 24 b, the insulating film 24 a functions as a filmwhich relieves damage to the oxide semiconductor film 19. Consequently,the insulating film 24 b can be formed using the high-frequency powerhaving a high power density while damage to the oxide semiconductor film19 is reduced.

When the insulating film 24 b is formed over the insulating film 24 aduring heat treatment, oxygen is moved to the oxide semiconductor film19 and oxygen vacancies in the oxide semiconductor film 19 can becompensated. Alternatively, when the insulating film 24 b is formed overthe insulating film 24 a and is then subjected to heat treatment, oxygenis moved to the oxide semiconductor film 19 and oxygen vacancies in theoxide semiconductor film 19 can be compensated. Consequently, the amountof oxygen vacancies in the oxide semiconductor film can be reduced.

When the oxide insulating film whose oxygen content is in excess of thatin the stoichiometric composition is provided over a back channel of theoxide semiconductor film 19 (a surface of the oxide semiconductor film19, which is opposite to a surface facing the gate electrode 15) withthe oxide insulating film which transmits oxygen provided therebetween,oxygen can be moved to the back channel side of the oxide semiconductorfilm 19, and oxygen vacancies on the back channel side can be reduced.

In the formation step of the insulating film 24 b, in the case where theoxide semiconductor film 19 is not damaged, the insulating film 24 a isnot necessarily provided and only the insulating film 24 b which is anoxide insulating film whose oxygen content is in excess of that in thestoichiometric composition may be provided as a protective film.

The nitride insulating film 25 is formed over the insulating film 24 b.By providing the nitride insulating film 25, the amount of hydrogen andammonia which are moved from the nitride insulating film to the oxidesemiconductor film 19 is small and the concentration of hydrogen andnitrogen in the oxide semiconductor film 19 can be reduced. Further, thenitride insulating film 25 is provided over the transistor 7, wherebyentry of water from the outside to the oxide semiconductor film 19 canbe suppressed. In other words, entry of hydrogen contained in water tothe oxide semiconductor film 19 can be suppressed. It is preferable thata blocking property of the nitride insulating film 25 against oxygen behigh because the movement of oxygen contained in the insulating film 24b to the outside can be suppressed and oxygen contained in theinsulating film 24 b can be moved to the oxide semiconductor film 19. Asa result, a shift of the threshold voltage in the negative direction canbe suppressed and variation in electrical characteristics can bereduced. Further, leakage current between a source and a drain of thetransistor, typically off-state current, can be reduced. In addition, achange in electrical characteristics due to a change over time or a BTphotostress test can be suppressed.

For the planarization film 27, an organic material, such as acrylicresin, epoxy resin, benzocyclobutene resin, polyimide, polyamide, or thelike can be used. Other than such organic materials, it is possible touse a silicone resin or the like. Note that the planarization film maybe formed by stacking a plurality of insulating films formed using thesematerials.

The organic material used for the planarization film 27 contains muchwater or gas than an inorganic insulating film, and the water and thegas are moved to the oxide semiconductor film by heat treatment in somecases. Further, the organic material easily transmits water from theoutside. Therefore the planarization film 27 is formed, wherebyelectrical characteristics in a transistor including the oxidesemiconductor film are changed by the water or gas, and the reliabilityof the transistor may be decreased.

Thus, like the transistor 7 in FIGS. 6A and 6B, the nitride insulatingfilm 25 which has a function of suppressing entry of water is preferablyprovided between the transistor 7 and the planarization film 27.

Further, the nitride insulating film 25 is preferably provided betweenthe insulating film 24 b and the planarization film 27 because theadhesion between the nitride insulating film 25 and the planarizationfilm 27 is improved.

For the conductive film 29, the material used for the pair of electrodes21 can be used as appropriate. As the conductive film 29, a conductivematerial having a light-transmitting property can be used, such asindium oxide which includes tungsten oxide, indium zinc oxide whichincludes tungsten oxide, indium oxide which includes titanium oxide,indium tin oxide which includes titanium oxide, indium tin oxide(hereinafter referred to as ITO), indium zinc oxide, or indium tin oxidewhich includes silicon oxide.

As the gate insulating film 18, an oxide insulating film whose oxygencontent is in excess of that in the stoichiometric composition may beused. An oxide insulating film whose oxygen content is in excess of thatin the stoichiometric composition is used as the gate insulating film18, whereby the interface state between the oxide semiconductor film 19and the gate insulating film 18 can be reduced, a shift of the thresholdvoltage in the negative direction can be suppressed, and variation inelectrical characteristics of the transistor can be reduced.

After the formation step of the pair of electrodes 21 over the oxidesemiconductor film 19 illustrated in FIG. 2C of Embodiment 1, an oxidesemiconductor film having few oxygen vacancies may be formed by exposingthe oxide semiconductor film 19 to plasma generated in an oxygenatmosphere and supplying oxygen to the oxide semiconductor film 19.Atmospheres of oxygen, ozone, dinitrogen monoxide, nitrogen dioxide, andthe like can be given as examples of oxygen atmospheres. Further, in theplasma treatment, the oxide semiconductor film 19 is preferred to beexposed to plasma generated with no bias applied to the substrate 11side. Consequently, the oxide semiconductor film 19 can be supplied withoxygen without being damaged; accordingly, the amount of oxygenvacancies in the oxide semiconductor film 19 can be reduced. Moreover,impurities remaining on the surface of the oxide semiconductor film 19due to the etching treatment for forming the pair of electrodes 21, forexample, a halogen such as fluorine or chlorine can be removed.

Through the above-described steps, a transistor with the improvedreliability, in which a change in electrical characteristics issuppressed, can be manufactured. Further, a transistor in which a changein electrical characteristics due to a change over time or a BTphotostress test is small can be manufactured. Typically a transistorwith the high reliability, in which variation in threshold voltage issmall, can be manufactured.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments andexamples.

(Embodiment 5)

In this embodiment, a structure of a gate insulating film which isdifferent from that in Embodiment 2 will be described with reference toFIGS. 7A and 7B.

FIGS. 7A and 7B are a top view and a cross-sectional view of atransistor 9 included in a semiconductor device. FIG. 7A is a top viewof the transistor 9, and FIG. 7B is a cross-sectional view taken alongdashed line A-B in FIG. 7A. Note that in FIG. 7A, the substrate 31, thebase insulating film 33, some components of the transistor 9 (e.g., aninsulating film 38 a, an insulating film 38 b, and the nitrideinsulating film 39), a planarization film 43, and the like are omittedfor simplicity.

The transistor 9 illustrated in FIGS. 7A and 7B includes the oxidesemiconductor film 34 formed over the base insulating film 33 and thepair of electrodes 35 in contact with the oxide semiconductor film 34.Further, the transistor 9 includes a gate insulating film 42 includingthe insulating film 38 a, the insulating film 38 b, and the nitrideinsulating film 39, and a gate electrode 41 overlapping with the oxidesemiconductor film 34 with the gate insulating film 42 providedtherebetween. A planarization film 43 covering the gate insulating film42 and the gate electrode 41 may be included. Moreover, a conductivefilm 45, which is in contact with one of the pair of electrodes 35through an opening 47 provided in the gate insulating film 42 and theplanarization film 43, may be included.

In the transistor 9 shown in this embodiment, the insulating film 38 ais formed in contact with the oxide semiconductor film 34. Theinsulating film 38 a is an oxide insulating film which transmits oxygen.As the insulating film 38 a, the insulating film 24 a in Embodiment 4can be used as appropriate.

By forming the oxide insulating film which transmits oxygen as theinsulating film 38 a, oxygen released from the oxide insulating filmprovided over the insulating film 38 a, whose oxygen content is inexcess of that in the stoichiometric composition, can be moved to theoxide semiconductor film 34 through the insulating film 38 a.

The insulating film 38 b is formed to be in contact with the insulatingfilm 38 a. The insulating film 38 b is an oxide insulating film whoseoxygen content is in excess of that in the stoichiometric composition.As the insulating film 38 b, the insulating film 24 b in Embodiment 4can be used as appropriate.

When the insulating film 38 b is formed over the insulating film 38 aduring heat treatment, oxygen is moved to the oxide semiconductor film34 and oxygen vacancies in the oxide semiconductor film 34 can becompensated. Alternatively, when the insulating film 38 b is formed overthe insulating film 38 a and is then subjected to heat treatment, oxygenis moved to the oxide semiconductor film 34 and oxygen vacancies in theoxide semiconductor film 34 can be compensated. Consequently, the amountof oxygen vacancies in the oxide semiconductor film can be reduced.

As the gate insulating film, an insulating film having few defects isused, whereby a shift of the threshold voltage in the negative directioncan be suppressed and variation in electrical characteristics of thetransistor can be reduced.

In the formation step of the insulating film 38 b, in the case where theoxide semiconductor film 34 is not damaged, the insulating film 38 a isnot necessarily provided and only the insulating film 38 b which is anoxide insulating film from which part of oxygen is released by heatingmay be provided.

The nitride insulating film 39 is formed over the insulating film 38 b.By providing the nitride insulating film 39, the amount of hydrogen andammonia which are moved from the nitride insulating film to the oxidesemiconductor film 34 is small and the concentration of hydrogen andnitrogen in the oxide semiconductor film 34 can be reduced. Further, thenitride insulating film 39 is provided over the transistor 9, wherebyentry of water from the outside to the oxide semiconductor film 34 canbe suppressed. In other words, entry of hydrogen contained in water tothe oxide semiconductor film 34 can be suppressed. It is preferable thata blocking property of the nitride insulating film 39 against oxygen behigh because the movement of oxygen contained in the insulating film 38b to the outside can be suppressed and oxygen contained in theinsulating film 38 b can be moved to the oxide semiconductor film 34. Asa result, a shift of the threshold voltage in the negative direction canbe suppressed and variation in electrical characteristics can bereduced. Further, leakage current between a source and a drain of thetransistor, typically off-state current, can be reduced. In addition, achange in electrical characteristics due to a change over time or a BTphotostress test can be suppressed.

The planarization film 43 can be formed using the material of theplanarization film 27, which is described in Embodiment 4, asappropriate.

Note that the nitride insulating film 39 is preferably provided betweenthe insulating film 38 b and the planarization film 43 because theadhesion between the nitride insulating film 39 and the planarizationfilm 43 is improved.

For the conductive film 45, the material of the conductive film 29,which is described in Embodiment 4, can be used as appropriate.

As in Embodiment 4, in this embodiment, after the formation step of thepair of electrodes 35 over the oxide semiconductor film 34 illustratedin FIG. 4B of Embodiment 2, an oxide semiconductor film having fewoxygen vacancies may be formed by exposing the oxide semiconductor film34 to plasma generated in an oxygen atmosphere and supplying oxygen tothe oxide semiconductor film 34. Consequently, the oxide semiconductorfilm 34 can be supplied with oxygen without being damaged; accordingly,the amount of oxygen vacancies in the oxide semiconductor film 34 can bereduced.

Through the above-described steps, a transistor with the improvedreliability, in which a change in electrical characteristics issuppressed, can be manufactured. Further, a transistor in which a changein electrical characteristics due to a change over time or a BTphotostress test is small can be manufactured. Typically a transistorwith the high reliability, in which variation in threshold voltage issmall, can be manufactured.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments andexamples.

(Embodiment 6)

A semiconductor device (also referred to as a display device) having adisplay function can be manufactured using the transistor examples ofwhich are shown in the above embodiments. Moreover, some or all of thedriver circuits which include the transistor can be formed over asubstrate where the pixel portion is formed, whereby a system-on-panelcan be obtained. In this embodiment, an example of a display deviceusing the transistor examples of which are shown in the aboveembodiments is described with reference to FIGS. 8A to 8C, FIGS. 9A and9B, FIG. 10, and FIGS. 11A to 11C. FIGS. 9A, 9B, and FIG. 10 arecross-sectional views illustrating cross-sectional structures takenalong chain line M-N in FIG. 8B.

In FIG. 8A, a sealant 905 is provided so as to surround a pixel portion902 provided over a first substrate 901, and the pixel portion 902 issealed with a second substrate 906. In FIG. 8A, a signal line drivercircuit 903 and a scan line driver circuit 904 each are formed using asingle crystal semiconductor or a polycrystalline semiconductor over asubstrate prepared separately, and mounted in a region different fromthe region surrounded by the sealant 905 over the first substrate 901.Further, various signals and potentials are supplied to the signal linedriver circuit 903, the scan line driver circuit 904, and the pixelportion 902 from flexible printed circuits (FPCs) 918 a and 918 b.

In FIGS. 8B and 8C, the sealant 905 is provided so as to surround thepixel portion 902 and the scan line driver circuit 904 which areprovided over the first substrate 901. The second substrate 906 isprovided over the pixel portion 902 and the scan line driver circuit904. Thus, the pixel portion 902 and the scan line driver circuit 904are sealed together with a display element by the first substrate 901,the sealant 905, and the second substrate 906. In FIGS. 8B and 8C, asignal line driver circuit 903 which is formed using a single crystalsemiconductor or a polycrystalline semiconductor over a substrateseparately prepared is mounted in a region different from the regionsurrounded by the sealant 905 over the first substrate 901. In FIGS. 8Band 8C, various signals and potentials are supplied to the signal linedriver circuit 903, the scan line driver circuit 904, and the pixelportion 902 from an FPC 918.

Although FIGS. 8B and 8C each show an example in which the signal linedriver circuit 903 is formed separately and mounted on the firstsubstrate 901, one embodiment of the present invention is not limited tothis structure. The scan line driver circuit may be separately formedand then mounted, or only part of the signal line driver circuit or partof the scan line driver circuit may be separately formed and thenmounted.

Note that a connection method of a separately formed driver circuit isnot particularly limited, and a chip on glass (COG) method, a wirebonding method, a tape automated bonding (TAB) method, or the like canbe used. FIG. 8A shows an example in which the signal line drivercircuit 903 and the scan line driver circuit 904 are mounted by a COGmethod. FIG. 8B shows an example in which the signal line driver circuit903 is mounted by a COG method. FIG. 8C shows an example in which thesignal line driver circuit 903 is mounted by a TAB method.

The display device includes in its category a panel in which a displayelement is sealed and a module in which an IC including a controller orthe like is mounted on the panel.

A display device in this specification refers to an image displaydevice, a display device, or a light source (including a lightingdevice). Further, the display device also includes the following modulesin its category: a module to which a connector such as an FPC or a TCPis attached; a module having a TCP at the tip of which a printed wiringboard is provided; and a module in which an integrated circuit (IC) isdirectly mounted on a display element by a COG method.

The pixel portion and the scan line driver circuit provided over thefirst substrate include a plurality of transistors and any of thetransistors which are described in the above embodiments can be used.

As the display element provided in the display device, a liquid crystalelement (also referred to as a liquid crystal display element) or alight-emitting element (also referred to as a light-emitting displayelement) can be used. A light emitting element includes, in its scope,an element whose luminance is controlled by current or voltage, andspecifically includes an inorganic electroluminescent (EL) element, anorganic EL element, and the like. Further, a display medium whosecontrast is changed by an electric effect, such as electronic ink, canbe used.

A light-emitting device shown in FIG. 9A includes a connection terminalelectrode 915 and a terminal electrode 916. The connection terminalelectrode 915 and the terminal electrode 916 are electrically connectedto a terminal included in the FPC 918 through an anisotropic conductiveagent 919.

The connection terminal electrode 915 is formed using the sameconductive film as a first electrode 930, and the terminal electrode 916is formed using the same conductive film as a pair of electrodes in eachof a transistor 910 and a transistor 911.

A light-emitting device shown in FIG. 9B includes a connection terminalelectrodes 915 a, 915 b, and a terminal electrode 916. The connectionterminal electrodes 915 a, 915 b, and the terminal electrode 916 areelectrically connected to a terminal included in the FPC 918 through ananisotropic conductive agent 919.

The connection terminal electrode 915 a is formed using the sameconductive film as a first electrode 930, the connection terminalelectrode 915 b is formed using the same conductive film as a secondelectrode 941, and the terminal electrode 916 is formed using the sameconductive film as a pair of electrodes in each of a transistor 910 anda transistor 911.

Further, as illustrated in FIG. 10, the semiconductor device includes aconnection terminal electrode 955 and a terminal electrode 916. Theconnection terminal electrode 955 and the terminal electrode 916 areelectrically connected to a terminal included in the FPC 918 through ananisotropic conductive agent 919.

The connection terminal electrode 955 is formed using the sameconductive film as a second electrode 951, and the terminal electrode916 is formed using the same conductive film as a pair of electrodes ineach of a transistor 910 and a transistor 911.

Each of the pixel portion 902 and the scan line driver circuit 904 whichare provided over the first substrate 901 includes a plurality oftransistors. FIGS. 9A, 9B, and FIG. 10 illustrate the transistor 910included in the pixel portion 902 and the transistor 911 included in thescan line driver circuit 904. In FIG. 9A, the protective film 26described in Embodiment 1 or an insulating film 924 corresponding to theprotective film 28 in Embodiment 4 is provided over the transistor 910and the transistor 911. In FIG. 9B, a planarization film 921 is furtherprovided over the insulating film 924. Note that an insulating film 923serves as a base film.

In this embodiment, any of the transistors described in the aboveembodiments can be used as the transistor 910 and the transistor 911.

Moreover, FIG. 10 shows an example in which a conductive film 917 isprovided over the insulating film 924 so as to overlap with a channelformation region of the oxide semiconductor film of the transistor 911for the driver circuit. In this embodiment, the conductive film 917 isformed using the same conductive film as the first electrode 930. Byproviding the conductive film 917 so as to overlap with the channelformation region of the oxide semiconductor film, the amount of changein the threshold voltage of the transistor 911 between before and aftera BT stress test can be further reduced. The conductive film 917 mayhave the same potential as or a potential different from that of thegate electrode of the transistor 911, and the conductive film 917 canserve as a second gate electrode. The potential of the conductive film917 may be GND, 0 V or in a floating state.

In addition, the conductive film 917 has a function of blocking anexternal electric field. In other words, the conductive film 917 has afunction of preventing an external electric field (particularly, afunction of preventing static electricity) from affecting the inside (acircuit portion including the transistor). Such a blocking function ofthe conductive film 917 can prevent a change in electricalcharacteristics of the transistor due to the influence of an externalelectric field such as static electricity. The conductive film 917 canbe used for any of the transistors described in the above embodiments.

In the display panel, the transistor 910 included in the pixel portion902 is electrically connected to a display element. There is noparticular limitation on the kind of the display element as long asdisplay can be performed, and various kinds of display elements can beused.

The first electrode and the second electrode (each of which may becalled a pixel electrode, a common electrode, a counter electrode, orthe like) for applying voltage to the display element may havelight-transmitting properties or light-reflecting properties, whichdepends on the direction in which light is extracted, the position wherethe electrode is provided, and the pattern structure of the electrode.

The first electrode 930, the second electrode 931, and the secondelectrode 941 can be formed using a light-transmitting conductivematerial such as indium oxide including tungsten oxide, indium zincoxide including tungsten oxide, indium oxide including titanium oxide,indium tin oxide including titanium oxide, indium tin oxide (hereinafterreferred to as ITO), indium zinc oxide, or indium tin oxide to whichsilicon oxide is added.

Alternatively, the first electrode 930, the second electrode 931, andthe second electrode 941 can be formed using one or more materialsselected from metals such as tungsten (W), molybdenum (Mo), zirconium(Zr), hafnium (Hf), vanadium (V), niobium (Nb), tantalum (Ta), chromium(Cr), cobalt (Co), nickel (Ni), titanium (Ti), platinum (Pt), aluminum(Al), copper (Cu), and silver (Ag); an alloy of any of these metals; anda nitride of any of these metals.

The first electrode 930, the second electrode 931, and the secondelectrode 941 can be formed using a conductive composition including aconductive macromolecule (also referred to as a conductive polymer). Asthe conductive high molecule, what is called a π-electron conjugatedconductive polymer can be used. For example, polyaniline or a derivativethereof, polypyrrole or a derivative thereof, polythiophene or aderivative thereof, a copolymer of two or more of aniline, pyrrole, andthiophene or a derivative thereof, and the like can be given.

An example of a liquid crystal display device using a liquid crystalelement as a display element is illustrated in FIGS. 9A and 9B. FIG. 9Aillustrates an example in which a vertical electric field method isemployed.

In FIG. 9A, a liquid crystal element 913 which is a display elementincludes the first electrode 930, a second electrode 931, and a liquidcrystal layer 908. Note that an insulating film 932 and an insulatingfilm 933 which serve as alignment films are provided so that the liquidcrystal layer 908 is provided therebetween. The second electrode 931 isprovided on the second substrate 906 side. The second electrode 931overlaps with the first electrode 930 with the liquid crystal layer 908provided therebetween.

FIG. 9B illustrates an example in which a fringe field switching (FFS)mode, which is one of horizontal electric field modes, is employed.

In FIG. 9B, a liquid crystal element 943 which is a display elementincludes the first electrode 930, the second electrode 941, and theliquid crystal layer 908 which are formed over the planarization film921. The second electrode 941 functions as the common electrode. Aninsulating film 944 is provided between the first electrode 930 and thesecond electrode 941. The insulating film 944 is formed using a siliconnitride film. An insulating film 932 and an insulating film 933 whichserve as alignment films are provided so that the liquid crystal layer908 is interposed therebetween.

A spacer 935 is a columnar spacer obtained by selective etching of aninsulating film and is provided in order to control the distance betweenthe first electrode 930 and the second electrode 931 (a cell gap).Alternatively, a spherical spacer may be used.

In the case where a liquid crystal element is used as the displayelement, a thermotropic liquid crystal, a low-molecular liquid crystal,a high-molecular liquid crystal, a polymer-dispersed liquid crystal, aferroelectric liquid crystal, an anti-ferroelectric liquid crystal, orthe like can be used. Such a liquid crystal material exhibits acholesteric phase, a smectic phase, a cubic phase, a chiral nematicphase, an isotropic phase, or the like depending on a condition.

Alternatively, a liquid crystal exhibiting a blue phase for which analignment film is unnecessary may be used. A blue phase is one of liquidcrystal phases, which is generated just before a cholesteric phasechanges into an isotropic phase while temperature of cholesteric liquidcrystal is raised. Since the blue phase appears only in a narrowtemperature range, a liquid crystal composition in which a chiralmaterial is mixed is used for the liquid crystal layer in order toimprove the temperature range. The liquid crystal composition whichincludes a liquid crystal showing a blue phase and a chiral agent has ashort response time of 1 msec or less, and has optical isotropy, whichmakes the alignment process unneeded and viewing angle dependence small.In addition, since an alignment film does not need to be provided andrubbing treatment is unnecessary, electrostatic discharge damage causedby the rubbing treatment can be prevented and defects and damage of theliquid crystal display device in the manufacturing process can bereduced. Thus, the productivity of the liquid crystal display device canbe increased.

The first substrate 901 and the second substrate 906 are fixed in placeby the sealant 925. As the sealant 925, an organic resin such as athermosetting resin or a photocurable resin can be used.

In the liquid crystal display device in FIG. 9A, the sealant 925 is incontact with a gate insulating film 922, and the planarization film 921is provided on an inner side than the sealant 925. Note that the gateinsulating film 922 is formed by stacking a silicon nitride film and asilicon oxynitride film. Further, when the insulating film 924 isselectively etched, it is preferable that the silicon nitride film beexposed by etching the silicon oxynitride film in the upper layer of thegate insulating film 922. As a result, the sealant 925 is in contactwith the silicon nitride film formed in the gate insulating film 922,and entry of water from the outside into the sealant 925 can besuppressed.

In the liquid crystal display device in FIG. 9B, the sealant 925 is incontact with the insulating film 924. The planarization film 921 isprovided on an inner side than the sealant 925 and the sealant 925 is incontact with the silicon nitride film on the surface of the insulatingfilm 924; thus, entry of water from the outside into the sealant 925 canbe suppressed.

The size of storage capacitor formed in the liquid crystal displaydevice is set considering the leakage current of the transistor providedin the pixel portion or the like so that charge can be held for apredetermined period. By using the transistor including thehighly-purified oxide semiconductor film, it is enough to provide astorage capacitor having a capacitance that is ⅓ or less, preferably ⅕or less of a liquid crystal capacitance of each pixel; therefore, theaperture ratio of a pixel can be increased.

In the display device, a black matrix (a light-blocking film), anoptical member (an optical substrate) such as a polarizing member, aretardation member, or an anti-reflection member, and the like areprovided as appropriate. For example, circular polarization may beobtained by using a polarizing substrate and a retardation substrate. Inaddition, a backlight, a side light, or the like may be used as a lightsource.

As a display method in the pixel portion, a progressive method, aninterlace method, or the like can be used. Further, color elementscontrolled in a pixel at the time of color display are not limited tothree colors: R, G, and B (R, G, and B correspond to red, green, andblue, respectively). For example, R, G, B, and W (W corresponds towhite), or R, G, B, and one or more of yellow, cyan, magenta, and thelike can be used. Further, the sizes of display regions may be differentbetween respective dots of color elements. The present invention is notlimited to the application to a display device for color display but canalso be applied to a display device for monochrome display.

FIGS. 11A to 11C illustrate an example of the display device in FIG. 9Ain which a common connection portion (pad portion) for electricallyconnecting to the second electrode 931 provided on the second substrate906 is formed over the first substrate 901.

Note that the contact hole in the pixel portion and the openings in thecommon connection portion are distinctively described because theirsizes differ considerably. In FIGS. 9A and 9B and FIGS. 11A to 11C, thepixel portion 902 and the common connection portion are not illustratedon the same scale. For example, the length of the chain line I-J in thecommon connection portion is about 500 μm, whereas the size of thetransistor of the pixel portion 902 is less than 50 μm; thus, the areaof the common connection portion is ten times or more as large as thatof the transistor. However, the scales of the pixel portion 902 and thecommon connection portion are changed in FIGS. 9A and 9B and FIGS. 11Ato 11C for simplification.

The common connection portion is provided in a position that overlapswith the sealant for bonding the first substrate 901 and the secondsubstrate 906, and is electrically connected to the second electrode 931through conductive particles contained in the sealant. Alternatively,the common connection portion is provided in a position that does notoverlap with the sealant (except for the pixel portion) and a pastecontaining conductive particles is provided separately from the sealantso as to overlap with the common connection portion, whereby the commonconnection portion is electrically connected to the second electrode931.

FIG. 11A is a cross-sectional view of the common connection portiontaken along a line I-J in the top view in FIG. 11B.

A common potential line 975 is provided over the gate insulating film922 and is formed using the same material and through the same steps asthe source electrode 971 or the drain electrode 973 of the transistor910 illustrated in FIGS. 9A and 9B.

Further, the common potential line 975 is covered with the insulatingfilm 924 and the planarization film 921, and a plurality of openings isincluded in the insulating film 924 and the planarization film 921 at aposition overlapping with the common potential line 975. These openingsare formed through the same steps as a contact hole which connects thefirst electrode 930 and one of the source electrode 971 and the drainelectrode 973 of the transistor 910.

Further, the common potential line 975 is connected to the commonelectrode 977 through the opening. The common electrode 977 is providedover the planarization film 921 and formed using the same material andthrough the same steps as the connection terminal electrode 915 and thefirst electrode 930 in the pixel portion.

In this manner, the common connection portion can be manufactured in thesame process as the switching element in the pixel portion 902.

The common electrode 977 is an electrode in contact with the conductiveparticles contained in the sealant, and is electrically connected to thesecond electrode 931 of the second substrate 906.

Alternatively, as illustrated in FIG. 11C, a common potential line 985may be formed using the same material and through the same steps as thegate electrode of the transistor 910.

In the common connection portion in FIG. 11C, the common potential line985 is provided under the gate insulating film 922, the insulating film924, and the planarization film 921, and a plurality of openings isincluded in the gate insulating film 922, the insulating film 924, andthe planarization film 921 at a position overlapping with the commonpotential line 985. These openings are formed by etching the insulatingfilm 924 and the planarization film 921 and further selectively etchingthe gate insulating film 922, which are the same steps as a contact holewhich connects the first electrode 930 and one of the source electrode971 and the drain electrode 973 of the transistor 910.

Further, the common potential line 985 is connected to the commonelectrode 987 in the opening portion. The common electrode 987 isprovided over the planarization film 921 and formed using the samematerial and through the same steps as the connection terminal electrode915 and the first electrode 930 in the pixel portion.

Note that in the liquid crystal display device of an FFS mode in FIG.9B, the common electrodes 977 and 987 are each connected to the secondelectrode 941.

Next, as the display element included in the display device, alight-emitting element utilizing electroluminescence can be used.Light-emitting elements utilizing electroluminescence are classifiedaccording to whether a light-emitting material is an organic compound oran inorganic compound. In general, the former is referred to as anorganic EL element, and the latter is referred to as an inorganic ELelement.

In the organic EL element, by application of voltage to a light-emittingelement, electrons and holes are separately injected from a pair ofelectrodes into a layer containing a light-emitting organic compound,and current flows. The carriers (electrons and holes) are recombined,and thus, the light-emitting organic compound is excited. Thelight-emitting organic compound returns to a ground state from theexcited state, thereby emitting light. Owing to such a mechanism, thislight-emitting element is referred to as a current-excitationlight-emitting element.

The inorganic EL elements are classified according to their elementstructures into a dispersion-type inorganic EL element and a thin-filminorganic EL element. The dispersion-type inorganic EL element has alight-emitting layer where particles of a light-emitting material aredispersed in a binder, and its light emission mechanism isdonor-acceptor recombination type light emission that utilizes a donorlevel and an acceptor level. The thin-film inorganic EL element has astructure where a light-emitting layer is sandwiched between dielectriclayers, which are further sandwiched between electrodes, and its lightemission mechanism is localized type light emission that utilizesinner-shell electron transition of metal ions. Note that an example ofan organic EL element as a light-emitting element is described here.

In order to extract light emitted from the light-emitting element, it isacceptable as long as at least one of a pair of electrodes istransparent. A transistor and a light-emitting element are formed over asubstrate. The light-emitting element can have a top emission structurein which light emission is extracted through the surface opposite to thesubstrate; a bottom emission structure in which light emission isextracted through the surface on the substrate side; or a dual emissionstructure in which light emission is extracted through the surfaceopposite to the substrate and the surface on the substrate side, and alight-emitting element having any of these emission structures can beused.

An example of a light-emitting device using a light-emitting element asthe display element is shown in FIG. 10. A light-emitting element 963which is a display element is electrically connected to the transistor910 provided in the pixel portion 902. Note that although the structureof the light-emitting element 963 is a stacked-layer structure of thefirst electrode 930, a light-emitting layer 961, and the secondelectrode 931, the structure is not limited thereto. The structure ofthe light-emitting element 963 can be changed as appropriate dependingon the direction in which light is extracted from the light-emittingelement 963, or the like.

A silicon nitride film 950 is provided between the planarization film921 and the first electrode 930. The silicon nitride film 950 is incontact with side surfaces of the planarization film 921 and theinsulating film 924. A partition wall 960 is provided over end portionsof the silicon nitride film 950 and the first electrode 930. A partitionwall 960 can be formed using an organic insulating material or aninorganic insulating material. It is particularly preferred that thepartition wall 960 be formed using a photosensitive resin material tohave an opening over the first electrode 930 so that a sidewall of theopening has an inclined surface with a continuous curvature.

The light-emitting layer 961 may be formed to have a single-layerstructure or a stacked-layer structure including a plurality of layers.

A protective layer may be formed over the second electrode 931 and thepartition wall 960 in order to prevent oxygen, hydrogen, moisture,carbon dioxide, or the like from entering the light-emitting element963. As the protective layer, a silicon nitride film, a silicon nitrideoxide film, an aluminum oxide film, an aluminum nitride film, analuminum oxynitride film, an aluminum nitride oxide film, a DLC film, orthe like can be formed. In addition, in a space which is sealed with thefirst substrate 901, the second substrate 906, and a sealant 936, afiller 964 is provided and sealed. It is preferred that, in this manner,the light-emitting element be packaged (sealed) with a protective film(such as a laminate film or an ultraviolet curable resin film) or acover material with high air-tightness and little degasification so thatthe panel is not exposed to the outside air.

As the sealant 936, an organic resin such as a thermosetting resin or aphotocurable resin, fitted glass including low-melting glass, or thelike can be used. The fritted glass is preferred because of its highbarrier property against impurities such as water and oxygen. Further,in the case where the fritted glass is used as the sealant 936, asillustrated in FIG. 10, the fitted glass is provided over the siliconnitride film 950, whereby adhesion of the silicon nitride film 950 tothe fritted glass becomes high and entry of water from the outside intothe sealant 936 can be prevented.

As the filler 964, as well as an inert gas such as nitrogen or argon, anultraviolet curable resin or a thermosetting resin can be used:polyvinyl chloride (PVC), an acrylic resin, polyimide, an epoxy resin, asilicone resin, polyvinyl butyral (PVB), ethylene vinyl acetate (EVA),or the like can be used. For example, nitrogen is used for the filler.

If necessary, an optical film such as a polarizing plate, a circularlypolarizing plate (including an elliptically polarizing plate), aretardation plate (a quarter-wave plate or a half-wave plate), or acolor filter may be provided as appropriate for a light-emitting surfaceof the light-emitting element. Further, a polarizing plate or acircularly polarizing plate may be provided with an anti-reflectionfilm. For example, anti-glare treatment by which reflected light can bediffused by projections and depressions on the surface so as to reducethe glare can be performed.

Further, an electronic paper in which electronic ink is driven can beprovided as the display device. The electronic paper is also referred toas an electrophoretic display device (an electrophoretic display) and isadvantageous in that it has the same level of readability as plainpaper, it has lower power consumption than other display devices, and itcan be made thin and lightweight.

Since the transistor is easily broken owing to static electricity or thelike, a protective circuit for protecting the driver circuit ispreferred to be provided. The protection circuit is preferred to beformed using a nonlinear element.

As described above, by using any of the transistors described in theabove embodiments, a highly reliable semiconductor device having adisplay function can be provided.

This embodiment can be implemented in appropriate combination with thestructures described in the other embodiments.

(Embodiment 7)

A semiconductor device having an image sensor function for reading dataof an object can be formed with the use of the transistor described inany of Embodiments 1 to 6.

An example of a semiconductor device having an image sensor function isillustrated in FIG. 12A. FIG. 12A illustrates an equivalent circuit of aphoto sensor, and FIG. 12B is a cross-sectional view illustrating partof the photo sensor.

In a photodiode 602, one electrode is electrically connected to aphotodiode reset signal line 658, and the other electrode iselectrically connected to a gate of a transistor 640. One of a sourceand a drain of the transistor 640 is electrically connected to a photosensor reference signal line 672, and the other of the source and thedrain thereof is electrically connected to one of a source and a drainof a transistor 656. A gate of the transistor 656 is electricallyconnected to a gate signal line 659, and the other of the source and thedrain thereof is electrically connected to a photo sensor output signalline 671.

Note that in circuit diagrams in this specification, a transistorincluding an oxide semiconductor film is denoted by a symbol “OS” sothat it can be identified as a transistor including an oxidesemiconductor film. In FIG. 12A, the transistor 640 and the transistor656 are each a transistor including an oxide semiconductor film, towhich the transistor described in any of Embodiments 1 to 6 can beapplied. In this embodiment, an example in which a transistor having astructure similar to that of the transistor 7 described in Embodiment 4is applied is described.

FIG. 12B is a cross-sectional view of the photodiode 602 and thetransistor 640 in the photosensor. The transistor 640 and the photodiode602 functioning as a sensor are provided over a substrate 601 (anelement substrate) having an insulating surface. A substrate 613 isprovided over the photodiode 602 and the transistor 640 with an adhesivelayer 608 interposed therebetween.

An insulating film 632, a planarization film 633, and a planarizationfilm 634 are provided over the transistor 640. The photodiode 602includes an electrode 641 b formed over the planarization film 633; afirst semiconductor film 606 a, a second semiconductor film 606 b, and athird semiconductor film 606 c over the electrode 641 b in this order;an electrode 642 which is provided over the planarization film 634 andelectrically connected to the electrode 641 b through the first to thirdsemiconductor films; and an electrode 641 a which is provided in thesame layer as the electrode 641 b and electrically connected to theelectrode 642.

The electrode 641 b is electrically connected to a conductive layer 643formed over the planarization film 634, and the electrode 642 iselectrically connected to a conductive film 645 through the electrode641 a. The conductive film 645 is electrically connected to a gateelectrode of the transistor 640, and thus the photodiode 602 iselectrically connected to the transistor 640.

Here, a pin photodiode in which a semiconductor film having p-typeconductivity type as the first semiconductor film 606 a, ahigh-resistance semiconductor film (i-type semiconductor film) as thesecond semiconductor film 606 b, and a semiconductor film having n-typeconductivity type as the third semiconductor film 606 c are stacked isillustrated as an example.

The first semiconductor film 606 a is a p-type semiconductor film andcan be formed using an amorphous silicon film containing an impurityelement imparting p-type conductivity. The first semiconductor film 606a is formed by a plasma CVD method with the use of a semiconductorsource gas containing an impurity element belonging to Group 13 (e.g.,boron (B)). As the semiconductor material gas, silane (SiH₄) may beused. Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or the likemay be used. Further alternatively, an amorphous silicon film which doesnot contain an impurity element may be formed, and then, an impurityelement may be introduced to the amorphous silicon film with use of adiffusion method or an ion injecting method. Heating or the like may beconducted after introducing the impurity element by an ion implantationmethod or the like in order to diffuse the impurity element. In thatcase, as a method of forming the amorphous silicon film, an LPCVDmethod, a chemical vapor deposition method, a sputtering method, or thelike may be used. The first semiconductor film 606 a is preferablyformed to a thickness greater than or equal to 10 nm and less than orequal to 50 nm.

The second semiconductor film 606 b is an i-type semiconductor film(intrinsic semiconductor film) and is formed using an amorphous siliconfilm. As for formation of the second semiconductor film 606 b, anamorphous silicon film is formed by a plasma CVD method with the use ofa semiconductor source gas. As the semiconductor material gas, silane(SiH₄) may be used. Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄,or the like may be used. The second semiconductor film 606 b may beformed by an LPCVD method, a vapor deposition method, a sputteringmethod, or the like. The second semiconductor film 606 b is preferablyformed to have a thickness greater than or equal to 200 nm and less thanor equal to 1000 nm.

The third semiconductor film 606 c is an n-type semiconductor film andis formed using an amorphous silicon film containing an impurity elementimparting n-type conductivity. The third semiconductor film 606 c isformed by a plasma CVD method with the use of a semiconductor source gascontaining an impurity element belonging to Group 15 (e.g., phosphorus(P)). As the semiconductor material gas, silane (SiH₄) may be used.Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or the like may beused. Further alternatively, an amorphous silicon film which does notcontain an impurity element may be formed, and then, an impurity elementmay be introduced to the amorphous silicon film with use of a diffusionmethod or an ion injecting method. Heating or the like may be conductedafter introducing the impurity element by an ion implantation method orthe like in order to diffuse the impurity element. In that case, as amethod of forming the amorphous silicon film, an LPCVD method, achemical vapor deposition method, a sputtering method, or the like maybe used. The third semiconductor film 606 c is preferably formed to havea thickness greater than or equal to 20 nm and less than or equal to 200nm.

The first semiconductor film 606 a, the second semiconductor film 606 b,and the third semiconductor film 606 c are not necessarily formed usingan amorphous semiconductor, and may be formed using a polycrystallinesemiconductor or a microcrystalline semiconductor (semi-amorphoussemiconductor: SAS).

In addition, the mobility of holes generated by the photoelectric effectis lower than the mobility of electrons. Therefore, a PIN photodiode hasbetter characteristics when a surface on the p-type semiconductor filmside is used as a light-receiving plane. Here, an example in which lightreceived by the photodiode 602 from a surface of the substrate 601, overwhich the pin photodiode is formed, is converted into electric signalsis described. Light from the semiconductor film having a conductivitytype opposite to that of the semiconductor film on the light-receivingplane is disturbance light; therefore, the electrode is preferablyformed using a light-blocking conductive film. Note that the n-typesemiconductor film side may alternatively be a light-receiving plane.

The insulating film 632, the planarization film 633, and theplanarization film 634 can be formed using an insulating material by asputtering method, a plasma CVD method, spin coating, dipping, spraycoating, a droplet discharge method (such as an inkjet method), screenprinting, offset printing, or the like depending on the material.

For the planarization films 633 and 634, for example, an organicinsulating material having heat resistance, such as polyimide, acrylicresin, a benzocyclobutene-based resin, polyamide, or epoxy resin, can beused. Other than such organic insulating materials, it is possible touse a single layer or stacked layers of a low-dielectric constantmaterial (a low-k material), a siloxane-based resin, phosphosilicateglass (PSG), borophosphosilicate glass (BPSG), or the like.

With detection of light that enters the photodiode 602, data on anobject to be detected can be read. Note that a light source such as abacklight can be used at the time of reading information on an object.

The structure, method, and the like described in this embodiment can beused in combination with structures, methods, and the like described inother embodiments and examples, as appropriate.

(Embodiment 8)

A semiconductor device disclosed in this specification can be applied toa variety of electronic devices (including game machines). Examples ofelectronic devices include a television set (also referred to as atelevision or a television receiver), a monitor of a computer or thelike, cameras such as a digital camera and a digital video camera, adigital photo frame, a mobile phone, a portable game machine, a portableinformation terminal, an audio reproducing device, a game machine (e.g.,a pachinko machine or a slot machine), a game console, and the like.Specific examples of these electronic devices are illustrated in FIGS.13A to 13C.

FIG. 13A illustrates a table 9000 having a display portion. In the table9000, a display portion 9003 is incorporated in a housing 9001 and animage can be displayed on the display portion 9003. Note that thehousing 9001 is supported by four leg portions 9002. Further, a powercord 9005 for supplying power is provided for the housing 9001.

The semiconductor device described in any of the above embodiments canbe used for the display portion 9003, so that the electronic device canhave high reliability.

The display portion 9003 has a touch-input function. When a user touchesdisplayed buttons 9004 which are displayed on the display portion 9003of the table 9000 with his/her finger or the like, the user can carryout operation of the screen and input of information. Further, when thetable may be made to communicate with home appliances or control thehome appliances, the table 9000 may function as a control device whichcontrols the home appliances by operation on the screen. For example,with use of the semiconductor device having an image sensor described inEmbodiment 7, the display portion 9003 can function as a touch panel.

Further, the screen of the display portion 9003 can be placedperpendicular to a floor with a hinge provided for the housing 9001;thus, the table 9000 can also be used as a television device. When atelevision device having a large screen is set in a small room, an openspace is reduced; however, when a display portion is incorporated in atable, a space in the room can be efficiently used.

FIG. 13B illustrates a television set 9100. In the television set 9100,a display portion 9103 is incorporated in a housing 9101 and an imagecan be displayed on the display portion 9103. Note that the housing 9101is supported by a stand 9105 here.

The television set 9100 can be operated with an operation switch of thehousing 9101 or a separate remote controller 9110. Channels and volumecan be controlled with an operation key 9109 of the remote controller9110 so that an image displayed on the display portion 9103 can becontrolled. Furthermore, the remote controller 9110 may be provided witha display portion 9107 for displaying data output from the remotecontroller 9110.

The television set 9100 illustrated in FIG. 13B is provided with areceiver, a modem, and the like. With the use of the receiver, thetelevision set 9100 can receive general TV broadcasts. Moreover, whenthe television set 9100 is connected to a communication network with orwithout wires via the modem, one-way (from a sender to a receiver) ortwo-way (between a sender and a receiver or between receivers)information communication can be performed.

The semiconductor device described in any of the above embodiments canbe used in the display portions 9103 and 9107, so that the televisionset and the remote controller can have high reliability.

FIG. 13C illustrates a computer, which includes a main body 9201, ahousing 9202, a display portion 9203, a keyboard 9204, an externalconnection port 9205, a pointing device 9206, and the like.

The semiconductor device described in any of the above embodiments canbe used for the display portion 9203, so that the computer can have highreliability.

FIGS. 14A and 14B illustrate a tablet terminal that can be folded. InFIG. 14A, the tablet terminal is opened, and includes a housing 9630, adisplay portion 9631 a, a display portion 9631 b, a display-modeswitching button 9034, a power button 9035, a power-saving-modeswitching button 9036, a clip 9033, and an operation button 9038.

The semiconductor device described in any of the above embodiments canbe used for the display portion 9631 a and the display portion 9631 b,so that the tablet terminal can have high reliability.

Part of the display portion 9631 a can be a touch panel region 9632 a,and data can be input by touching operation keys 9638 that aredisplayed. Although a structure in which a half region in the displayportion 9631 a has only a display function and the other half regionalso has a touch panel function is shown as an example, the displayportion 9631 a is not limited to the structure. However, the structureof the display portion 9631 a is not limited to this, and all the areaof the display portion 9631 a may have a touch panel function. Forexample, all the area of the display portion 9631 a can display keyboardbuttons and serve as a touch panel while the display portion 9631 b canbe used as a display screen.

In the display portion 9631 b, as in the display portion 9631 a, part ofthe display portion 9631 b can be a touch panel region 9632 b. When afinger, a stylus, or the like touches the place where a button 9639 forswitching to keyboard display is displayed in the touch panel, keyboardbuttons can be displayed on the display portion 9631 b.

Touch input can be performed concurrently on the touch panel regions9632 a and 9632 b.

The display-mode switching button 9034 allows switching between alandscape mode and a portrait mode, color display and black-and-whitedisplay, and the like. With the power-saving-mode switching button 9036for switching to power-saving mode, the luminance of display can beoptimized in accordance with the amount of external light at the timewhen the tablet is in use, which is detected with an optical sensorincorporated in the tablet. The tablet terminal may include anotherdetection device such as a sensor for detecting orientation (e.g., agyroscope or an acceleration sensor) in addition to the optical sensor.

Although the display portion 9631 a and the display portion 9631 b havethe same display area in FIG. 14A, one embodiment of the presentinvention is not limited to this example. The display portion 9631 a andthe display portion 9631 b may have different areas or different displayquality. For example, one of them may be a display panel that candisplay higher-definition images than the other.

FIG. 14B illustrates the tablet terminal folded, which includes thehousing 9630, a solar battery 9633, and a charge and discharge controlcircuit 9634. Note that FIG. 14B shows an example in which the chargeand discharge control circuit 9634 includes a battery 9635 and a DCDCconverter 9636.

Since the tablet terminal can be folded in two, the housing 9630 can beclosed when the tablet terminal is not in use. Thus, the displayportions 9631 a and 9631 b can be protected, thereby providing a tabletterminal with high endurance and high reliability for long-term use.

The tablet terminal illustrated in FIGS. 14A and 14B can also have afunction of displaying various kinds of data (e.g., a still image, amoving image, and a text image), a function of displaying a calendar, adate, the time, or the like on the display portion, a touch-inputfunction of operating or editing data displayed on the display portionby touch input, a function of controlling processing by various kinds ofsoftware (programs), and the like.

The solar battery 9633, which is attached on the surface of the tabletterminal, supplies electric power to a touch panel, a display portion,an image signal processor, and the like. Note that the solar battery9633 can be provided on one or two surfaces of the housing 9630, so thatthe battery 9635 can be charged efficiently. When a lithium ion batteryis used as the battery 9635, there is an advantage of downsizing or thelike.

The structure and operation of the charge and discharge control circuit9634 illustrated in FIG. 14B are described with reference to a blockdiagram of FIG. 14C. The solar battery 9633, the battery 9635, the DCDCconverter 9636, a converter 9637, switches SW1 to SW3, and the displayportion 9631 are illustrated in FIG. 14C, and the battery 9635, the DCDCconverter 9636, the converter 9637, and the switches SW1 to SW3correspond to the charge and discharge control circuit 9634 illustratedin FIG. 14B.

First, an example of operation in the case where power is generated bythe solar battery 9633 using external light is described. The voltage ofpower generated by the solar battery 9633 is raised or lowered by theDCDC converter 9636 so that a voltage for charging the battery 9635 isobtained. When the display portion 9631 is operated with the power fromthe solar battery 9633, the switch SW1 is turned on and the voltage ofthe power is raised or lowered by the converter 9637 to a voltage neededfor operating the display portion 9631. In addition, when display on thedisplay portion 9631 is not performed, the switch SW1 is turned off anda switch SW2 is turned on so that charge of the battery 9635 may beperformed.

Here, the solar battery 9633 is shown as an example of a powergeneration means; however, there is no particular limitation on a way ofcharging the battery 9635, and the battery 9635 may be charged withanother power generation means such as a piezoelectric element or athermoelectric conversion element (Peltier element). For example, thebattery 9635 may be charged with a non-contact power transmission modulewhich is capable of charging by transmitting and receiving power bywireless (without contact), or another charging means may be used incombination.

The structure, method, and the like described in this embodiment can beused in combination with structures, methods, and the like described inother embodiments, as appropriate.

EXAMPLE 1

In this example, results of evaluating a nitride insulating film whichcan be used for the transistor of one embodiment of the presentinvention are described. In detail, results of evaluating the number ofhydrogen molecules, ammonia molecules, and water molecules which arereleased by heating are described.

First, a method for forming the evaluated samples is described. Theformed samples each have a structure 1 or a structure 2.

A silicon nitride film 993 was formed over a silicon wafer 991 by aplasma CVD method using formation conditions which can be used for thenitride insulating film 25 described in Embodiment 1 (see FIG. 1B), sothat the sample having the structure 1 was formed (see FIG. 15A).

The silicon nitride film 993 was formed using three conditions which area condition 1, a condition 2, and a condition 3. The sample formed usingthe condition 1 is referred to as a sample A1. The sample formed usingthe condition 2 is referred to as a sample A2. The sample formed usingcondition 3 is referred to as a sample A3. The samples A1 to A3 eachhave the silicon nitride film 993 with a thickness of 50 nm.

The condition 1 was as follows: the temperature of the silicon wafer 991was 220° C.; the source gas was silane, nitrogen, and ammonia with aflow rate of 50 sccm, 5000 sccm, and 100 sccm, respectively; thepressure of the treatment chamber was 200 Pa; and the high-frequencypower supplied to parallel plate electrodes was 27.12 MHz and 1000 W(the power density was 1.6×10⁻¹ W/cm²). The flow ratio of nitrogen toammonia was 50.

The condition 2 was the same as the condition 1 except that thehigh-frequency power supplied to parallel plate electrodes was 150 W(the power density was 2.5×10⁻² W/cm²).

The condition 3 was as follows: the temperature of the silicon wafer 991was 220° C.; the source gas was silane, nitrogen, and ammonia with aflow rate of 30 sccm, 1500 sccm, and 1500 sccm, respectively; thepressure of the treatment chamber was 200 Pa; and the high-frequencypower supplied to parallel plate electrodes was 27.12 MHz and 150 W (thepower density was 2.5×10⁻² W/cm²). The flow ratio of nitrogen to ammoniawas 1.

TDS analyses were performed on the samples A1 to A3. In each of thesamples, the silicon wafer 991 was heated at 65° C. or higher and 610°C. or lower.

The peaks of the curves shown in the results obtained from TDS appeardue to release of atoms or molecules contained in the analyzed samples(in this example, the samples A1 to A3) to the outside. The total numberof the atoms or molecules released to the outside corresponds to theintegral value of the peak. Thus, with the degree of the peak intensity,the number of the atoms or molecules contained in the silicon nitridefilm can be evaluated.

FIGS. 16A to 16C and FIGS. 17A and 17B show the results of the TDSanalyses on the samples A1 to A3. FIG. 16A is a graph of the amount of areleased gas which has a M/z of 2, typically hydrogen molecules, againstthe substrate temperature. FIG. 16B is a graph of the amount of areleased gas which has a M/z of 18, typically water molecules, againstthe substrate temperature. FIG. 16C is a graph of an amount of releasedhydrogen molecules calculated from an integral value of a peak of acurve in FIG. 16A. FIG. 17A is a graph of the amount of a released gaswhich has a M/z of 17, typically ammonia molecules, against thesubstrate temperature. FIG. 17B is a graph of an amount of releasedammonia molecules calculated from an integral value of a peak of a curvein FIG. 17A. In these TDS analyses, the lower limit of detection ofhydrogen molecules was 1×10²¹ molecules/cm³, and, the lower limit ofdetection of ammonia molecules was 2×10²⁰ molecules/cm³.

As shown in FIG. 16A, the TDS intensity of hydrogen molecules of thesample A2 is higher than that of the sample A1 and that of the sampleA3. As shown in FIG. 16C, the amount of released hydrogen molecules ofthe sample A2 against the substrate temperature is approximately fivetimes that of the sample A1 and the sample A3. As shown in FIG. 16B, inthe samples A1 to A3, a peak indicating the release of water moleculesis seen when the temperature of each substrate was in the range fromhigher than or equal to 100° C. to lower than or equal to 200° C. Notethat only in the sample A3, a sharp peak was detected in the range.

In contrast, as shown in FIG. 17A, the TDS intensity of ammoniamolecules of the sample A3 is higher than that of the sample A1 and thesample A2. As shown in FIG. 17B, the amount of released ammoniamolecules of the sample A3 against the substrate temperature is at leastapproximately greater than or equal to 16 times that of the sample A1and the sample A2. The amount of released ammonia molecules of thesample A2 is less than or equal to the lower limit of detection.

Next, the structure 2 which was employed to some of the formed samplesis described. A silicon oxynitride film 995 was formed over the siliconwafer 991 by a plasma CVD method using formation conditions which can beused for the insulating film 24 b described in Embodiment 4 (see FIG.6B), and the silicon nitride film 993 was formed over the siliconoxynitride film 995 in a manner similar to the structure 1, so that thesample having the structure 2 was formed (see FIG. 15B).

In each of the samples having the structure 2, in order to evaluate aneffect of suppressing movement of water in the silicon nitride film 993,the silicon oxynitride film 995 is a silicon oxynitride film whoseoxygen content is in excess of that in the stoichiometric composition.FIGS. 19A and 19B show the results of TDS analyses on samples in each ofwhich only the silicon oxynitride film 995 having a thickness of 400 nmwas formed over a silicon wafer. In each of the samples, the siliconwafer 991 was heated at 70° C. or higher and 570° C. or lower. FIG. 19Ais a graph of the amount of a released gas which has a M/z of 32,typically oxygen molecules, against the substrate temperature. FIG. 19Bis a graph of the amount of a released gas which has a M/z of 18,typically water molecules, against the substrate temperature. Thesilicon oxynitride film whose oxygen content is in excess of that in thestoichiometric composition contains not only oxygen (see FIG. 19A) butalso water (see FIG. 19B); thus, by evaluating an amount of releasedwater molecules against the substrate temperature of the samples A4 toA6 having the structure 2, whether or not the silicon nitride film 993has an effect of suppressing movement of water can be evaluated.

The formation conditions of the silicon oxynitride film 995 was asfollows: the temperature of the silicon wafer 991 was 220° C.; thesource gas was silane and nitrogen monoxide with a flow rate of 160 sccmand 4000 sccm, respectively; the pressure of the treatment chamber was200 Pa; and the high-frequency power supplied to parallel plateelectrodes was 27.12 MHz and 1500 W (the power density was 2.5×10⁻¹W/cm²). The thickness of the silicon oxynitride film 995 was 400 nm.

In the samples having the structure 2, the silicon nitride film 993 wasformed using the three conditions, which are the condition 1, thecondition 2, and the condition 3. The sample which has the structure 2and is formed using the condition 1 is referred to as a sample A4. Thesample which has the structure 2 and is formed using the condition 2 isreferred to as a sample A5. The sample which has the structure 2 and isformed using the condition 3 is referred to as a sample A6. The samplesA4 to A6 each have the silicon nitride film 993 with a thickness of 50nm. The details of the conditions 1 to 3 are the same as those of thestructure 1.

TDS analyses were performed on the samples A4 to A6 in order to evaluatean effect of suppressing movement of water. In each of the samples, thesilicon wafer 991 was heated at 70° C. or higher and 580° C. or lower.

FIGS. 18A and 18B show the results of the TDS analyses on the samples A4to A6 having the structure 2. FIG. 18A is a graph of an amount ofreleased hydrogen molecules against the substrate temperature. FIG. 18Bis a graph of an amount of released water molecules against thesubstrate temperature.

As shown in FIG. 18A, the TDS intensity of hydrogen molecules of thesample A5 is higher than that of the sample A4 and that of the sampleA6. As shown in FIG. 18B, a minor peak is seen in the TDS intensity ofwater molecules; however, large difference is not seen among the samplesA4 to A6.

The samples A4 to A6 having the structure 2 each have a very lowintensity of a peak indicating the release of water molecules despitethe presence of the silicon oxynitride film 995 containing water. Thus,with the formation conditions of the samples A4 to A6, an insulatingfilm which can suppress movement of water.

However, the sample A2 including a silicon nitride film using acondition similar to that of the sample A5 has a large number ofreleased hydrogen molecules, and the sample A3 has a large number ofreleased ammonia molecules. In a transistor including an oxidesemiconductor, when hydrogen and nitrogen are contained in an oxidesemiconductor film, electrons serving as carriers are generated in theoxide semiconductor film and the transistor becomes normally on. Thus,hydrogen molecules and ammonia molecules which are sources for supplyingnitrogen are both impurities which change electrical characteristics ofa transistor. For example, in the sample A3, the amount of releasedammonia molecules is large, which means that there are many nitrogensources, and by forming such an insulating film over a transistor or ina gate insulating film of a transistor, the transistor becomes normallyon.

Thus, the nitride insulating film which releases a small number ofhydrogen molecules and ammonia molecules, such as the silicon nitridefilm formed under the condition 1 used in the sample A1 and the sampleA4 is provided over a transistor including an oxide semiconductor film.As a result, a transistor in which a change in electricalcharacteristics is suppressed or a transistor whose reliability isimproved can be manufactured. Further, as a gate insulating film of thetransistor including the oxide semiconductor film, a nitride insulatingfilm which releases a small number of released hydrogen molecules andreleased ammonia molecules, such as a silicon nitride film formed underthe condition 1 using in the samples A1 and A4, is provided, whereby atransistor in which variation in electrical characteristics issuppressed or a transistor with improved reliability can bemanufactured.

Next, transistors including silicon nitride films formed under theconditions 1 to 3 were manufactured and the Vg-Id characteristics weremeasured.

A manufacturing process of a transistor included in each of a sample B1,a sample B2, and a sample B3 is described. In this example, the processis described with reference to FIGS. 2A to 2D.

First, as illustrated in FIG. 2A, a glass substrate was used as thesubstrate 11, and the gate electrode 15 was formed over the substrate11.

A 100 nm-thick tungsten film was formed by a sputtering method, a maskwas formed over the tungsten film by a photolithography process, andpart of the tungsten film was etched with the use of the mask, so thatthe gate electrode 15 was formed.

Next, the gate insulating film 18 was formed over the gate electrode 15.

As the gate insulating film 18, a stacked layer including a 50-nm-thicksilicon nitride film and a 200-nm-thick silicon oxynitride film werestacked. The silicon nitride film was formed in the followingconditions: silane and nitrogen were supplied at 50 sccm and 5000 sccm,respectively, into a treatment chamber of a plasma CVD apparatus; thepressure of the treatment chamber was adjusted to 60 Pa; and power of150 W was supplied with the use of a 27.12 MHz high-frequency powersource. The silicon oxynitride film was formed in the followingconditions: silane and dinitrogen monoxide were supplied at 20 sccm and3000 sccm, respectively, into the treatment chamber of the plasma CVDapparatus; the pressure of the treatment chamber was adjusted to 40 Pa;and power of 100 W was supplied with the use of a 27.12 MHzhigh-frequency power source. Note that each of the silicon nitride filmand the silicon oxynitride film was formed at a substrate temperature of350° C.

Next, the oxide semiconductor film 19 overlapping with the gateelectrode 15 with the gate insulating film 18 provided therebetween wasformed.

Here, an IGZO film which was a CAAC-OS film was formed over the gateinsulating film 18 by a sputtering method, a mask is formed over theIGZO film by a photolithography process, and the IGZO film was partlyetched using the mask. Then, the etched IGZO film was subjected to heattreatment, so that the oxide semiconductor film 19 was formed. Note thatthe IGZO film formed in this example has a thickness of 35 nm.

The IGZO film was formed in such a manner that a sputtering target whereIn:Ga:Zn=1:1:1 (atomic ratio) was used, argon and oxygen were suppliedas a sputtering gas into a treatment chamber of a sputtering apparatusat a flow rate of 50 sccm for each, the pressure in the treatmentchamber was controlled to be 0.6 Pa, and direct-current power of 5 kWwas supplied. Note that the IGZO film was formed at a substratetemperature of 170° C.

Next, water, hydrogen, and the like contained in the oxide semiconductorfilm were released by heat treatment. Here, heat treatment at 450° C.for one hour in a nitrogen atmosphere was performed, and then heattreatment at 450° C. for one hour in an atmosphere of nitrogen andoxygen was performed.

For the structure obtained through the steps up to here, FIG. 2B can bereferred to.

Next, after the gate electrode was exposed by etching a part of the gateinsulating film 18 (not illustrated), the pair of electrodes 21 incontact with the oxide semiconductor film 19 was formed as illustratedin FIG. 2C.

A conductive film was formed over the gate insulating film 18 and theoxide semiconductor film 19, a mask was formed over the conductive filmby a photolithography process, and the conductive film was partly etchedusing the mask, so that the pair of electrodes 21 was formed. Note thatas the conductive film, a 400-nm-thick aluminum film was formed over a50-nm-thick tungsten film, and a 100-nm-thick titanium film was formedover the aluminum film.

Next, after the substrate was moved to a treatment chamber under reducedpressure and heated at 220° C., the substrate was moved to a treatmentchamber filled with dinitrogen monoxide. Then, the oxide semiconductorfilm 19 was exposed to oxygen plasma which was generated in such amanner that an upper electrode provided in the treatment chamber wassupplied with high-frequency power of 150 W with the use of a 27.12 MHzhigh-frequency power source.

Next, the insulating film 23 was formed in succession over the oxidesemiconductor film 19 and the pair of electrodes 21 without exposure tothe atmosphere after the above plasma treatment. A 50-nm-thick firstsilicon oxynitride film and a 400-nm-thick second silicon oxynitridefilm were stacked.

The first silicon oxynitride film was formed by a plasma CVD methodunder the following conditions: silane with a flow rate of 30 sccm anddinitrogen monoxide with a flow rate of 4000 sccm were used as a sourcegas, the pressure in a treatment chamber was 40 Pa, the substratetemperature was 220° C., and high-frequency power of 150 W was suppliedto parallel-plate electrodes.

The second silicon oxynitride film was formed by a plasma CVD methodunder the following conditions: silane with a flow rate of 160 sccm anddinitrogen monoxide with a flow rate of 4000 sccm were used as a sourcegas, the pressure in the treatment chamber was 200 Pa, the substratetemperature was 220° C., and high-frequency power of 1500 W was suppliedto the parallel-plate electrodes. Under the above conditions, it ispossible to form a silicon oxynitride film whose oxygen content is inexcess of that in the stoichiometric composition and from which part ofoxygen is released by heating.

Next, water, hydrogen, and the like were released from the insulatingfilm 23 by heat treatment. Here, the heat treatment was performed in anatmosphere of nitrogen and oxygen at 350° C. for one hour.

Next, as illustrated in FIG. 2D, the nitride insulating film 25 wasformed over the insulating film 23.

In the sample B1, as the nitride insulating film 25, the silicon nitridefilm was formed under the condition 1 of the sample A1.

In the sample B2, as the nitride insulating film 25, the silicon nitridefilm was formed under the condition 2 of the sample A2.

In the sample B3, as the nitride insulating film 25, the silicon nitridefilm was formed under the condition 3 of the sample A3.

Next, although not illustrated, parts of the insulating film 23 and thenitride insulating film 25 were etched, and openings which expose partof the pair of electrodes were formed.

Next, a planarization film (not illustrated) was formed over the nitrideinsulating film 25. Here, the nitride insulating film 25 was coated witha composition, and exposure and development were performed, so that aplanarization film having an opening through which the pair ofelectrodes is partly exposed was formed. Note that as the planarizationfilm, a 1.5-μm-thick acrylic resin was formed. Then, heat treatment wasperformed. The heat treatment was performed at a temperature of 250° C.in a nitrogen atmosphere for one hour.

Next, a conductive film connected to part of the pair of electrodes isformed (not illustrated). Here, a 100-nm-thick ITO film containingsilicon oxide was formed by a sputtering method.

Through these steps, transistors in the samples B1 to B3 weremanufactured.

Next, Vg-Id characteristics of the transistors in the samples B1 to B3were measured.

Next, a pressure cooker test (PCT) was performed as the accelerated lifetest to evaluate moisture resistance. In the PCT in this example, thesamples B1 to B3 were held for 15 hours under the following conditions:the temperature was 130° C., the humidity was 85%, and the pressure was0.23 MPa.

FIGS. 20A to 20C, FIGS. 21A to 21C, and FIGS. 22A to 22C show Vg-Idinitial characteristics of the transistors included in the samples B1 toB3 and Vg-Id characteristics of the transistors included in the samplesB1 to B3 after the pressure cooker test.

Note that in each of the samples, Vg-Id characteristics of a transistor1 whose channel length (L) is 2 μm and channel width (W) is 50 μm and atransistor 2 whose channel length (L) is 6 μm and channel width (W) is50 μm were measured. The initial characteristics of the transistors 1 inthe samples B1 to B3 are shown in FIG. 20A, FIG. 21A, and FIG. 22A, theinitial characteristics of the transistors 2 in the samples B1 to B3 areshown in FIG. 20B, FIG. 21B, and FIG. 22B, and the Vg-Id characteristicsof the transistors 2 in the samples B1 to B3 after the pressure cookertest are shown in FIG. 20C, FIG. 21C, and FIG. 22C. Further, in each ofthe samples, 24 transistors having the same structure were manufacturedon the substrate.

According to the Vg-Id characteristics shown in FIG. 21A, thetransistors do not have switching characteristics. Further, according tothe Vg-Id characteristics shown in FIG. 22A, variation in thresholdvoltage of the transistors is large. However, according to the Vg-Idcharacteristics shown in FIG. 20A, it is found that the transistors hasfavorable switching characteristics and variation in threshold voltageof the transistors is small.

It is found that variation in threshold voltage of the transistor in theinitial characteristics of the Vg-Id characteristics shown in FIG. 20Band FIG. 22B is smaller than that in the initial characteristics of theVg-Id characteristics shown in FIG. 21B.

The Vg-Id characteristics shown in FIG. 20C have more favorableswitching characteristics than the Vg-Id characteristics after thepressure cooker test shown in FIG. 21C and FIG. 22C.

For the above reasons, a nitride insulating film which releases a smallnumber of hydrogen molecules and ammonia molecules is formed over atransistor, whereby a shift of threshold voltage in the negativedirection can be reduced and the reliability of the transistor can beimproved.

Next, a plurality of samples was manufactured by forming the nitrideinsulating film 25 through the same process as the samples B1 to B3 inthis example and under a condition other than the conditions 1 to 3. Ineach of the samples, 24 transistors having the same structure wereformed on the substrate, and the Vg-Id initial characteristics of thetransistors were compared to one another. Note that in each of thetransistors, the channel length (L) is 2 μm and the channel width (W) is50 μm.

FIG. 23 shows a relation between an amount of released hydrogenmolecules and an amount of released ammonia molecules from the nitrideinsulating film 25 and the Vg-Id initial characteristics of thetransistors in the plurality of samples in which the nitride insulatingfilm 25 is formed under a condition of the samples B1 to B3 or acondition other than the conditions 1 to 3.

In FIG. 23, the horizontal axis indicates the amount of hydrogenmolecules released from the nitride insulating film 25 and the verticalaxis indicates the amount of ammonia molecules released from the nitrideinsulating film 25. Further, in FIG. 23, circles indicate that thedifference between the maximum threshold voltage and the minimumthreshold voltage (Vth_max−Vth_min) in the 24 transistors on thesubstrate is less than or equal to 1 V. Further, triangles indicate thatVth_max−Vth_min is greater than 1 V and less than or equal to 3 V.Further, crosses indicate that Vth_max−Vth_min is greater than 3 V.

In FIG. 23, crosses are not plotted in the region where the amount ofreleased hydrogen molecules from the nitride insulating film 25 issmaller than 5×10²¹ molecules/cm³ and the amount of released ammoniamolecules from the nitride insulating film 25 is smaller than 1×10²²molecules/cm³. Accordingly, it is found that a nitride insulating filmwhich releases hydrogen molecules less than 5×10²¹ molecules/cm³ andammonia molecules less than 1×10²² molecules/cm³ is provided over atransistor, whereby variation in threshold voltage of the transistor canbe reduced. Moreover, a shift of the threshold voltage in the negativedirection can be suppressed.

REFERENCE NUMERALS

1: transistor, 3: transistor, 5: transistor, 7: transistor, 9:transistor, 11: substrate, 15: gate electrode, 18: gate insulating film,19: oxide semiconductor film, 21: electrode, 23: insulating film, 24 a:insulating film, 24 b: insulating film, 25: nitride insulating film, 26:protective film, 27: planarization film, 28: protective film, 29:conductive film, 30: opening, 31: substrate, 33: base insulating film,34: oxide semiconductor film, 35: electrode, 37: insulating film, 38 a:insulating film, 38 b: insulating film, 39: nitride insulating film, 40:gate insulating film, 41: gate electrode, 42: gate insulating film, 43:planarization film, 45: conductive film, 47: opening, 61: gateelectrode, 601: substrate, 602: photodiode, 606 a: semiconductor film,606 b: semiconductor film, 606 c: semiconductor film, 608: adhesivelayer, 613: substrate, 632: insulating film, 633: planarization film,634: planarization film, 640: transistor, 641 a: electrode, 641 b:electrode, 642: electrode, 643: conductive film, 645: conductive film,656: transistor, 658: photodiode reset signal line, 659: gate signalline, 671: photo sensor output signal line, 672: photo sensor referencesignal line, 901: substrate, 902: pixel portion, 903: signal line drivercircuit, 904: scan line driver circuit, 905: sealant, 906: substrate,908: liquid crystal layer, 910: transistor, 911: transistor, 913: liquidcrystal element, 915: connection terminal electrode, 915 a: connectionterminal electrode, 915 b: connection terminal electrode, 916: terminalelectrode, 917: conductive film, 918: FPC, 918 b: FPC, 919: anisotropicconductive agent, 921: planarization film, 922: gate insulating film,923: insulating film, 924: insulating film, 925: sealant, 930:electrode, 931: electrode, 932: insulating film, 933: insulating film,935: spacer, 936: sealant, 941: electrode, 943: liquid crystal element,944: insulating film, 950: silicon nitride film, 951: electrode, 955:connection terminal electrode, 960: partition wall, 961: light-emittinglayer, 963: light-emitting element, 964: filler, 971: source electrode,973: drain electrode, 975: common potential line, 977: common electrode,985: common potential line, 987: common electrode, 991: silicon wafer,993: silicon nitride film, 995: silicon oxynitride film, 9000: table,9001: housing, 9002: leg portion, 9003: display portion, 9004: displayedbutton, 9005: power cord, 9033: clip, 9034: switching button, 9035:power button, 9036: switching button, 9038: operation button, 9100:television set, 9101: housing, 9103: display portion, 9105: stand, 9107:display portion, 9109: operation key, 9110: remote controller, 9201:main body, 9202: housing, 9203: display portion, 9204: keyboard, 9205:external connection port, 9206: pointing device, 9630: housing, 9631:display portion, 9631 a: display portion, 9631 b: display portion, 9632a: region, 9632 b: region, 9633: solar battery, 9634: charge anddischarge control circuit, 9635: battery, 9636: DCDC converter, 9637:converter, 9638: operation key, 9639: button.

This application is based on Japanese Patent Application serial no.2012-147703 filed with Japan Patent Office on Jun. 29, 2012, the entirecontents of which are hereby incorporated by reference.

The invention claimed is:
 1. A method for manufacturing a semiconductordevice comprising the steps of: forming a first gate electrode; forminga gate insulating film over the first gate electrode; forming an oxidesemiconductor film over the gate insulating film, the oxidesemiconductor film overlapping with the first gate electrode; forming apair of electrodes in contact with the oxide semiconductor film; andforming a nitride insulating film over the oxide semiconductor film by aplasma CVD method using a source gas containing nitrogen and ammonia,wherein in the step of forming the nitride insulating film, a flow ratioof the nitrogen to the ammonia is greater than or equal to 5 and lessthan or equal to 50, and wherein in the case where the nitrideinsulating film is analyzed by thermal desorption spectroscopy, hydrogenmolecules less than 5×10²¹ molecules/cm³ and ammonia molecules less than1×10²² molecules/cm³ are released.
 2. The method for manufacturing asemiconductor device according to claim 1, wherein the flow ratio of thenitrogen to the ammonia is greater than or equal to 10 and less than orequal to
 50. 3. The method for manufacturing a semiconductor deviceaccording to claim 1, further comprising the step of: forming an oxideinsulating film over the oxide semiconductor film before the step offorming the nitride insulating film, wherein the oxide insulating filmis in contact with part of the oxide semiconductor film.
 4. The methodfor manufacturing a semiconductor device according to claim 1, furthercomprising a step of: performing a heat treatment on the oxidesemiconductor film in an atmosphere containing nitrogen and oxygen. 5.The method for manufacturing a semiconductor device according to claim1, wherein the source gas further contains a deposition gas containingsilicon.
 6. The method for manufacturing a semiconductor deviceaccording to claim 1, further comprising the step of: forming a secondgate electrode over the nitride insulating film, the second gateelectrode overlapping with the first gate electrode.
 7. A method formanufacturing a semiconductor device comprising the steps of: forming aninsulating film; forming an oxide semiconductor film over the insulatingfilm; forming a pair of electrodes in contact with the oxidesemiconductor film; forming a gate insulating film comprising a nitrideinsulating film over the oxide semiconductor film; and forming a gateelectrode over the gate insulating film, wherein the nitride insulatingfilm is formed by a plasma CVD method using a source gas containingnitrogen and ammonia, wherein in the step of forming the nitrideinsulating film, a flow ratio of the nitrogen to the ammonia is greaterthan or equal to 5 and less than or equal to 50, and wherein in the casewhere the nitride insulating film is analyzed by thermal desorptionspectroscopy, hydrogen molecules less than 5×10²¹ molecules/cm³ andammonia molecules less than 1×10²² molecules/cm³ are released.
 8. Themethod for manufacturing a semiconductor device according to claim 7,wherein the insulating film comprises an oxide insulating film, andwherein oxygen is added to the oxide insulating film before the step offorming the oxide semiconductor film.
 9. The method for manufacturing asemiconductor device according to claim 7, wherein the flow ratio of thenitrogen to the ammonia is greater than or equal to 10 and less than orequal to
 50. 10. The method for manufacturing a semiconductor deviceaccording to claim 7, wherein the gate insulating film further comprisesan oxide insulating film, and wherein the oxide insulating film isbetween the oxide semiconductor film and the nitride insulating film.11. The method for manufacturing a semiconductor device according toclaim 7, further comprising the step of: performing a heat treatment onthe oxide semiconductor film in an atmosphere containing nitrogen andoxygen.
 12. The method for manufacturing a semiconductor deviceaccording to claim 7, wherein the source gas further contains adeposition gas containing silicon.
 13. A method for manufacturing asemiconductor device comprising the steps of: forming a first gateelectrode; forming a gate insulating film comprising a first nitrideinsulating film over the first gate electrode; forming an oxidesemiconductor film over the gate insulating film, the oxidesemiconductor film overlapping with the first gate electrode; forming apair of electrodes electrically connected to the oxide semiconductorfilm; and forming a second nitride insulating film over the oxidesemiconductor film, wherein the first nitride insulating film and thesecond nitride insulating film are each formed by a plasma CVD methodusing a source gas containing nitrogen and ammonia, wherein in each ofthe step of forming the first nitride insulating film and the step offorming the second nitride insulating film, a flow ratio of the nitrogento the ammonia is greater than or equal to 5 and less than or equal to50, and wherein in the case where each of the first nitride insulatingfilm and the second nitride insulating film are analyzed by thermaldesorption spectroscopy, hydrogen molecules less than 5×10²¹molecules/cm³ and ammonia molecules less than 1×10²² molecules/cm³ arereleased.
 14. The method for manufacturing a semiconductor deviceaccording to claim 13, wherein the flow ratio of the nitrogen to theammonia is greater than or equal to 10 and less than or equal to
 50. 15.The method for manufacturing a semiconductor device according to claim13, further comprising the step of: forming an oxide insulating filmover the oxide semiconductor film before the step of forming the secondnitride insulating film, wherein the oxide insulating film is in contactwith part of the oxide semiconductor film.
 16. The method formanufacturing a semiconductor device according to claim 13, furthercomprising a step of: performing a heat treatment on the oxidesemiconductor film in an atmosphere containing nitrogen and oxygen. 17.The method for manufacturing a semiconductor device according to claim13, wherein the source gas further contains a deposition gas containingsilicon.
 18. The method for manufacturing a semiconductor deviceaccording to claim 13, further comprising the step of: forming a secondgate electrode over the second nitride insulating film, the second gateelectrode overlapping with the first gate electrode.